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Disease is a term for any condition that impairs the normal functioning of an organism or body. Although plants and animals also contract diseases, by far the most significant disease-related areas of interest are those conditions that afflict human beings. They can be divided into three categories: intrinsic, or coming from within the body; extrinsic, or emerging from outside it; and of unknown origin. Until the twentieth century brought changes in the living standards and health care of industrialized societies, extrinsic diseases were the greater threat; today, however, diseases of intrinsic origin are much more familiar. Among them are stress-related diseases, autoimmune disorders, cancers, hereditary diseases, glandular conditions, and conditions resulting from malnutrition. There are also illnesses, such as Alzheimer's disease, whose causes remain essentially unknown.


Classifying Diseases

Any condition that impairs the normal functioning of an organism can be called a disease. In the human organism, as in all others, there are certain basic requirements, which in the human body include the need for a certain proper amount of oxygen, acidity, salinity (salt content), nutrients, and so on. These conditions must all be maintained within a very narrow range, and any deviation can bring about disease.

Diseases can be classified into three general groups. There are conditions that are infectious, or extrinsic, meaning that they are caused by an infection through which a virus, bacterium, or other parasite enters the body. Infectious diseases, infections, and the immune system that usually protects us against them are discussed elsewhere in this book. Our attention in the present context will be devoted to the other two broad categoriesnoninfectious, or intrinsic, diseases and diseases of unknown origin.


There are several basic varieties of intrinsic disease, or conditions that are neither contagious nor communicable. These varieties are listed in the next few paragraphs. The essay Noninfectious Diseases includes a discussion of other systems for classifying diseases of either the intrinsic or the extrinsic variety.

Hereditary diseases: diseases that are genetic, meaning that they are passed down from generation to generation. An example, discussed in Noninfectious Diseases, is hemophilia. Heredity is not a "cause," and some of the diseases of unknown origin may be transmitted from parent to offspring. Some forms of cancer are hereditary as well, as are other conditions discussed elsewhere in this book. (See Nonifectious Diseases, Mutation, and Heredity.)

Glandular diseases: Conditions involving a glandthat is, a cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. Examples include diabetes mellitus, examined in Noninfectious Diseases, as well as various kidney and liver diseases, among them, hepatitis and jaundice. Goiter, a swelling in the neck area caused by a diet poor in iodine, is both a glandular and a dietary condition, a fact that illustrates the overlap between disease types.

Dietary diseases: These are all illnesses that relate to nutrient deficiencieseither an overall lack of adequate nutrition (i.e., malnutrition) or the absence of a key nutrient. Examples include pellagra, scurvy, and rickets, all of which are vitamin deficiencies, as well as kwashiorkor, which brings about a swollen belly and is caused by a lack of protein. Vitamin deficiencies are discussed in Vitamins, and kwashiorkor and other varieties of malnutrition are examined in Nutrients and Nutrition.

Cancers: Cancer is not just one disease but some 100 conditions. Its two main characteristics are uncontrolled growth of diseased cells in the human body and migration of the disease from the original site to distant sites within the body. If the spread is not controlled, cancer can result in death. (See Noninfectious Diseases for more.)

Stress-related diseases: Some heart conditions are hereditary or glandular, but quite a few diseases of the heart and circulatory system are exacerbated by stress. Examples include heart murmurs, hardening of the arteries, and varicose veins. We will examine heart disease and the general effects of stress shortly.

Autoimmune diseases: This is a particularly terrifying category of disease, because it involves a rejection of the body itself by the body's own immune system. Autoimmune diseases, examples of which include lupus and rheumatoid arthritis, are discussed in The Immune System.


Finally, there are diseases for which there is no known cause. In some cases, it is possible that heredity, diet, or some other aspect of human existence has a role, but it is not certain. And even if, say, heredity plays a part, the exact hereditary factors are not established. In any case, many of the categories of disease we have listed do not amount to "causes," but rather are types of disease. Moreover, some diseases classifiable in one of the listed categories also belong in the ranks of the diseases with unknown causes. For instance, many autoimmune diseases are mysterious to scientists. Likewise, chronic fatigue syndrome, considered a disease of unknown origin, is obviously a stress-related disorder, while fibromyalgia, characterized by sore muscles and tissues, may be stress-related as well. Two brain diseases of unknown origin, Creutzfeldt-Jakob and Alzheimer's disease, are discussed near the conclusion of this essay.


A Changing Threat

At one time the diseases that posed the greatest threat to human survival were infectious ones, such as the Black Death (actually a combination of bubonic and pneumonic plague), which killed about a third of Europe's population during the period from 1347 to 1351. Plagues or epidemics, in fact, are among the persistent themes in history, punctuating the fall of empires and the rise of others.

A plague that struck the eastern Roman (Byzantine) Empire in the sixth century, for instance, brought an end to a plan by the great Justinian I to reconquer the Italian peninsula and restore Roman rule in western Europe. It also spelled the beginning of the end of Byzantine glory (though the empire hung on until 1453) and opened the way for the rise of Islam and Muslim influence over the Mediterranean. Thus, the course of history up to the present day, including the events of the European Middle Ages, the Crusades, and even the modern-day conflict between the West and Islamic terrorists, can be traced in part back to a plague in about a.d. 540.

Wherever people have gathered in large numbers, infectious diseases have arisen. Smallpox and chicken pox, cholera and malaria, diphtheria and scarlet fever, influenza and poliothese and many other diseases have threatened the very survival of whole populations, bringing about a collective death toll that dwarfs that of twentieth-century wars and genocide. Yet it was in the twentieth centuryironically, the era when humans discovered the capacity to kill themselves in truly frightening numbers through world wars, nuclear weaponry, and totalitarian social experimentsthat the threat of infectious diseases began to recede.

Thanks to successful vaccination programs, many infectious diseases are largely a thing of the past. This is true even of smallpox, a scourge that effectively ended in 1978 thanks to a United Nations inoculation program, but which reemerged as a potential threat of biological terrorism in the hands of political terrorists, such as the Islamic terrorist Osama bin Laden. It would be difficult for bin Laden's al-Qaeda organization to acquire samples of the virus, however; they are stored in only four or five laboratories worldwide (kept there for the purpose of making more vaccine if needed) and remain under heavy guard.


Rather than infectious diseases, the much greater threat today is in the form of intrinsic diseases, or ones that are neither communicable nor contagious. The leading causes of death in the United States are as follows:

  • Heart disease
  • Cancer
  • Stroke
  • Chronic obstructive lung diseases (e.g., emphysema)
  • Accidents (motor vehicle or other)
  • Pneumonia and influenza
  • Diabetes mellitus
  • Suicide
  • Kidney disease
  • Chronic liver disease and cirrhosis

Note that the one item on the list that is not an intrinsic disease and is not disease-related at all: accidents. After that is the first extrinsic disease entry on the list, number 6, pneumonia and the closely related condition influenza. Number 8, of course, is not related to diseaseat least not physical disease. The high incidence of suicide, with 11.1 such deaths per 100,000 population, probably reflects the fact that the United States is an industrialized, wealthy nation. Ironically, people who are eking out a living, struggling for survival, are far less likely to end it all voluntarily.

Also reflective of America's high level of development is the overwhelming preponderance of intrinsic, noninfectious diseases on the list. Unquestionably, the greatest threat to human health today takes the form of noninfectious diseases, such as heart disease, cancer, and diseases of the circulatory system. This is true only in the industrialized world, however: whereas only about 25% of all patients who visit doctors in the United States do so because of infectious diseases, more than two-thirds of all deaths worldwide are caused by infectious diseases, such as malaria.

Stress and Heart Disease

Stress, simply put, is a condition of mental or physical tension brought about by internal or external pressures. Many events can cause stress: something as simple as taking a test or driving through rush-hour traffic or as traumatic as the death of a loved one or contracting a serious illness. Stress may be short-lived, as when facing a particular deadline, or it may be the ongoing, crippling stress related to a job that is slowly killing the victim.

People who experience severe traumas, such as soldiers in combat, may experience a condition called post-traumatic stress disorder (PTSD). This condition first came to public attention after World War I, a war that completely dwarfed all preceding conflicts in its intensity and brutality. Formerly bright-eyed, optimistic youths came home behaving like madmen or nervous wrecks, and soon the condition gained the nickname shell shock. (Actually, shell shock dated back more than 50 years, to what might be regarded as the first modern war in the Westthe first "total war" involving relatively sophisticated weaponry and a fully engaged citizenry: America's Civil War, from which combatants returned home with a condition known as "soldier's heart.")


Whereas PTSD has a distinct psychological dimension, in many stress-related diseases there is not as obvious a link between mental states and bodily disorders. Nonetheless, it is clear that stress kills. Some of the physical signs of stress are a dry mouth and throat, headaches, indigestion, tremors, muscle tics, insomnia, and a tightness of the muscles in the shoulders, neck, and back. Emotional signs of stress include tension, anxiety, and depression. During stress, heart rate quickens, blood pressure increases, and the body releases the hormone adrenaline, which speeds up the body's metabolism. Stress may disrupt homeostasis, an internal bodily system of checks and balances, leading to a weakening of immunity.

Diseases and conditions associated with stress include adult-onset diabetes (see Noninfectious Diseases), ulcers, high blood pressure, asthma, migraine headaches, cancer, and even the common cold. The last, of course, is an infectious illness, but because stress impairs the immune system, it can leave a person highly susceptible to infection. Furthermore, medical researchers have determined that long-term stress causes the accumulation of fat, starch, calcium, and other substances in the linings of the blood vessels. This condition ultimately results in heart disease.


The human heart weighs just 10.5 oz. (300 g), but it contracts more than 100,000 times a day to drive blood through about 60,000 mi. (96,000 km) of vessels. An average heart will pump about 1,800 gal. (6,800 l) of blood each day. With exercise, that amount may increase as much as six times. In an average lifetime the heart will pump about 100 million gal. (380 million l) of blood. The heart is divided into four chambers: the two upper atria and the two lower ventricles. The wall that divides the right and left sides of the heart is the septum. Movement of blood between chambers and in and out of the heart is controlled by valves that allow transit in only one direction.

Given its importance to human life, it follows that heart disease is an extremely serious condition. Among the many illnesses that fall under the general heading of heart disease is congenital heart disease, a term for any defect in the heart that is present at birth. About one of every 100 infants is born with some sort of heart abnormality, the most common form being the atrial septal defect, in which an opening in the septum allows blood from the right and left atria to mix.

Coronary heart disease, also known as coronary artery disease, is the most common form of heart disease. A condition termed arteriosclerosis, in which there is a thickening of the artery walls, or a variety of arteriosclerosis known as atherosclerosis results when fatty material, such as cholesterol, accumulates on an artery wall. This forms plaque, which obstructs blood flow. When the obstruction occurs in one of the main arteries leading to the heart, the heart does not receive enough blood and oxygen, and its muscle cells begin to die.

Creutzfeldt-Jakob Disease

A particularly frightening category of unexplained diseases includes those that attack and destroy the brain. Among them are two conditions named after German scientists: the psychiatrists Alfons Maria Jakob (1884-1931) and Hans Gerhard Creutzfeldt (1885-1964) and the neurologist Alois Alzheimer (1864-1915). Creutzfeldt-Jakob disease, fortunately, is a rare condition. The disease, first described by the two doctors in the 1920s, initially shows itself with the loss of memory, and within a few weeks it progresses to visual problems, loss of coordination, and seizure-like muscular jerking. Death usually follows within a year.

It appears that Creutzfeldt-Jakob disease ensues when a certain protein in the brain, known as prion protein, changes into an abnormal form. As to what causes that change, scientists remain in the dark. The disease attacks about one of a million people worldwide, and victims are typically about 50-75 years of age. During the 1990s something strange happened: the disease began affecting relatively large numbers of young people in the United Kingdom. A 1996 report of British medical experts, however, linked the surge in Creutzfeldt-Jakob cases to what might be considered a dietary condition: bovine spongiform encephalopathy, or mad cow disease, contracted from eating cattle with a form of prion disease. The only way to contract such a condition, however, is by eating the brain or spinal cord of an affected cow, something that could only happen in the case of hamburger or sausage, in which one does not always know what one is getting. The cows themselves got the disease from eating feed tainted with by-products of other cows, and as a result of the outbreak, Great Britain issued wide-ranging controls prohibiting the production of feed containing any materials from cows. (These particular feed-production practices were never common in the United States.)

Alzheimer's Disease

Whereas Creutzfeldt-Jakob disease is a little-known condition, Alzheimer's disease is all too familiar to the families of the more than four million sufferers in America today. Note the reference to the families rather than the victims themselves: one of the most devastating aspects of Alzheimer's disease is the patient's progressive loss of contact with reality, such that a patient in an advanced stage does not even know that he or she has the disease. A progressive brain disease that brings about mental deterioration, Alzheimer's disease is signaled by symptoms that include increasingly poor memory, personality changes, and a loss of concentration and judgment. Although most victims are older 65 years, Alzheimer's is not a normal result of aging. Up until the 1970s people assumed that physical and mental decline were normal and unavoidable features of old age and dismissed such cases of deterioration as "senility." Yet as early as 1906, Alzheimer himself discovered evidence that pointed in a different direction.

In that year Alzheimer was studying a 51-year-old woman whose personality and mental abilities were obviously deteriorating. She forgot things, became paranoid, acted strangely, and just over four years after he began working with her, she died. Following an autopsy, Alzheimer examined sections of her brain under a microscope and noted deposits of an unusual substance in her cerebral cortexthe outer, wrinkled layer of the brain, where many of the higher brain functions, such as memory, speech, and thought, originate. The substance Alzheimer saw under the microscope is now known to be a protein called beta-amyloid. About 75 years later scientists and physicians began to recognize a strong link between "senility" and the condition Alzheimer had identified. Since then, the public has become more aware of the disease, especially since Alzheimer's disease has stricken such well-known figures as the former president Ronald Reagan (1911-) and the actress Rita Hayworth (1918-1987).


A slight decline in short-term memory (as opposed to long-term memories of childhood and the like) is typical even in healthy elderly adults, but the memory loss seen in Alzheimer's disease is much more severe. As years pass, memory loss becomes greater, and personality and behavioral changes occur. Later symptoms include disorientation, confusion, speech impairment, restlessness, irritability, and the inability to care for oneself. Although victims may remain physically healthy for years, the progressive decline of their mental faculties is ultimately fatal: eventually, the brain loses the ability to control basic physical functions, such as swallowing. Persons with Alzheimer's disease typically live between five and ten years after diagnosis, although improvements in health care in recent years have enabled some victims to survive for 15 years or even longer.

Improvements in health care also may help explain the fact that the numbers of Alzheimer victims are growing. Medical discoveries of the twentieth century served to prolong life greatly, such that there are far more people alive today who are 65 years of age or older than there were in 1900. More accurate reporting no doubt plays a part as well. Whereas about 2.5 million cases were reported throughout the 1970s, by the end of the twentieth century there were some four million living Alzheimer victims, and by the mid-twenty-first century that number is expected to climb to the range of 13 million if physicians do not find a cure. Meanwhile, Alzheimer's causes the deaths of more than 100,000 American adults each year and costs $80-90 billion annually in health-care expenses.


It is not a simple procedure to diagnose Alzheimer's disease, and despite all the medical progress since the time of Alois Alzheimer, the "best" method for determining whether someone has the condition is hardly a good one. The only possible physical procedure for definitively diagnosing Alzheimer's disease is to open the skull and remove a sample of brain tissue for microscopic examination. This is rarely done, of course, because brain surgery is far too drastic a procedure for simply obtaining a sample of tissue.

The immediate cause of Alzheimer's is the death of brain cells and a decrease in the connections between those cells that survive. But what causes that? Many scientists today believe that the presence of beta-amyloid protein is a cause in itself, while others maintain that the appearance of the protein is simply a response to some other, still unknown phenomenon. Researchers have found that a small percentage of Alzheimer cases apparently are induced by genetic mutations, but most cases result from unknown factors. Various risk factors have been identified, but they are not the same as causes; rather, a risk factor simply means that if a person has x, he or she is more likely to have y. Risk factors for Alzheimer's include exposure to toxins, head trauma (former president Reagan suffered a serious head injury before the onset of Alzheimer's disease), Down syndrome (a genetic disorder that causes mental retardation), age, and even gender (women are more likely than men to suffer from Alzheimer's disease).

Familial Alzheimer's disease, an inherited form, accounts for about 10% of cases. Approximately 100 families in the world are known to have rare genetic mutations that are linked with early onset of symptoms, and some of these families have an aggressive form of the disease in which symptoms appear before age 40. The remaining 90% of cases may be caused by various combinations of genetic and as yet undefined environmental factors.


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A general term for any condition that impairs the normal functioning of an organism.


A term for a disease that is communicable or contagious and comes from outside the body. Compare with intrinsic.


A cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste.


A term for a disease that is not communicable or contagious and comes from inside the body. Compare with extrinsic.

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disease can be broadly defined as any illness or sickness that impairs or disrupts the normal functioning of the human body. Definitions of disease, however, have varied over time and place, and diagnostic categories vary. We will be concerned here with concepts of disease in the Western medical tradition. Medicine itself can be defined as ‘the science and art concerned with the cure, alleviation, and prevention of disease, and with the restoration and preservation of health’. Thus the definition, treatment, and prevention, of disease is intimately associated with the development of medicine itself, and several specialist areas of medicine are specifically concerned with broad aspects of disease. For example nosology, the classification of diseases, was for many centuries the cornerstone of medicine; therapeutics is the treatment of disease; diagnosis is the art the physician uses to determine from the patients' signs and symptoms what the underlying causative mechanism may be; epidemiology is the study of disease in populations.

The Hippocratic tradition maintained that diseases were physiological, arising from an imbalance between the four humours, the correct balance of which maintained health. Each individual had a unique humoral balance which could be easily disrupted by conditions such as cold, biting winds, poor air, or injudicious eating. It was Paracelsus (c.1492–1541) who provided an alternative to such classical ideas by suggesting that disease was the product of active agents independent of the human patient. The English Hippocrates, Thomas Sydenham (1624–89), also believed that diseases were specific entities, which might be manifest in variable ways in individual patients, but which could be recognized by observant clinicians. Sydenham also advocated that specific remedies could be applied to each such disease, his favourite example being the prescription of Peruvian bark for intermittent fever or ague. It was during the eighteenth century that the classification of diseases became a dominant part of medicine, such taxonomies often being based on the presenting symptoms. Increasing attention to pathology and cellular mechanisms during the nineteenth century provided additional criteria whereby diseases could be described, recognized, and treated.

An increasing array of diagnostic techniques have become available during the twentieth century, and these have made it possible for even more diseases, syndromes, and conditions to be classified, and the International Classification of Disease goes some way to providing international standardization in the categorization of modern disease.

Disease categories

One broad classification divides diseases into two principle categories, defining disease as primarily congenital (present at birth) or acquired subsequently. The acquired category can be further subdivided to include infectious, neoplastic, traumatic, and degenerative diseases, which are not necessarily mutually exclusive. Modern concepts and definitions have obscured many of these classificatory boundaries, and occupational, nutritional and deficiency, autoimmune and allergic, and psychiatric diseases can now be included in contemporary nosologies. As the molecular mechanisms of diseases and their causations are increasingly understood, so classifications are increasingly becoming blurred and overlapping, and many diseases are now recognized as being multi-causal. The following sections will give brief overviews of some broad classes of disease.

Genetics and diseases

Hereditary diseases may be passed down from generation to generation, but are not necessarily genetic disorders unless determined by one or more genes. The experiments on peas by Gregor Mendel (1822–84), on the transfer of characteristics from generation to generation, established the basic principles of heredity. Mendel and his successors determined a number of factors to define genetic inheritance, including its occurrence in known proportions amongst relatives, but not in unrelated individuals, such as in-laws. Applied to human conditions, some diseases were now identified as hereditary, including haemophilia and sickle-cell anaemia — although it had been recognized for centuries that some diseases ‘ran in the family’.

congenital abnormalities are present at birth, though some do not immediately become apparent. They are not necessarily inherited, many in fact being environmentally determined during intrauterine life. It is now known that many common diseases, including heart disease, insulin dependent diabetes, some forms of psychiatric illnesses, and autoimmune diseases, have a genetic component. But these susceptibilities are often triggered by some environmental influence, and represent a complex interaction between nature (genetic makeup) and nurture (external influences). Some forms of cancer are increasingly recognized as having a major genetic component. The genetics of some diseases, such as cystic fibrosis, beta-thalassaemia, and Duchenne's muscular dystrophy, are now well known, and it is possible to screen parents, and unborn fetuses, to detect genetic abnormalities, and to offer termination of an affected fetus. The development of in utero gene therapy offers the hope of treatment, whilst modern medicine can do a great deal to maintain individuals who, in earlier periods, would have died because of their genetic constitution.

Infectious diseases

infectious diseases caused by microorganisms or parasites have been powerful forces in shaping human history. As early humans formed hunter–gatherer societies in about 3000 bc and began to contain and domesticate wild animals, so they became susceptible to the infections carried by the animals with which they now shared their living space. Smallpox, distemper, and measles are amongst the diseases known to have entered human populations at this time. The devastating effects of these diseases gradually became ameliorated as immunity built up in communities — major epidemics (and pandemics) were often caused by the movement of communities where such diseases were endemic into non-immune populations. Urbanization provided fresh opportunities for infectious diseases to flourish and to decimate populations — close living conditions encouraged the transfer of infections; migration into the cities from rural communities, and the dependency of urban populations on the countryside for food, provided fresh avenues for infection. Until almost the end of the nineteenth century, sustaining cities was a constant problem, and the large metropolitan areas of Western civilization were known to be centres of disease, malnutrition and starvation, and ultimately death. Diseases such as smallpox, syphilis, typhoid fever, and whooping cough were all endemic and accounted for a high infant mortality — estimates have suggested for example that the mortality rate for children under 5 was as high as 50% for much of the nineteenth century in the English city of Manchester. By the latter half of that century, the experiments of Robert Koch (1843–1910) in Germany and Louis Pasteur (1822–95) in France, amongst others, increasingly provided evidence that microganisms were the cause of several infectious diseases. Efforts were made to utilize this knowledge in the manufacture of vaccines, preparations of modified or killed bacteria that could be administered to produce a mild form of the disease and to confer immunity. Although the precise mechanisms of how such immunity was created were unknown, a number of therapeutic substances were developed, the biggest breakthrough coming at the end of the nineteenth century with the appearance of serum anti-toxins. Since then, increasing understanding of the underlying cellular mechanisms of immunity and of the biology of microorganisms; the growth and development of the pharmaceutical industry, especially the discovery of antibiotics; and the development of public health measures to prevent and treat infectious diseases, have led to a notable decline in mortality and morbidity from such diseases in the Western world. The same cannot be said for the developing world, and concerns grew at the end of the twentieth century about resistance to antibiotics, the appearance of new infectious killers such as HIV (which causes AIDS), and the re-emergence of drug-resistant forms of diseases such as TB.

Autoimmune diseases

autoimmune diseases occur when elements of the immune system, normally responsible for recognizing and attacking ‘non-self’ cells — such as the microorganisms that cause infectious diseases — fail to distinguish between ‘self’ and ‘non-self’. Such cellular attacks on healthy constituent parts of the body can contribute to a variety of disorders, including myasthenia gravis, some thyroid disorders, and rheumatoid arthritis. There is growing evidence that conditions such as diabetes and multiple sclerosis also have an autoimmune component.

Diseases associated with food: deficiencies, excesses, and intolerances

Deficiency (nutritional) diseases arise from lack of one or more essential nutritional component, such as a vitamin or mineral, in the diet, or because of the body's inability to digest, absorb, or utilize particular nutrients. A nutritional deficiency can also occur if the body's metabolism is abnormal or if essential elements are excessively excreted. Historically, deficiency diseases have arisen in populations forced, by war or famine, to abandon their traditional diets, or by the adoption, perhaps for religious reasons, of a restrictive diet. Expeditions into new territories have always been vulnerable to dietary diseases because of the difficulty of carrying adequate supplies. The most notable example was that of scurvy, and the development of its treatment by eating citrus fruits, which occurred long before the rationale was understood — namely that this corrected a vitamin C deficiency due to lack of fresh fruit and vegetables. Subtle changes to farming or cooking methods can also lead to unexpected deficiencies. One of the best known examples is the use of white (huskless) rice instead of brown (husked) rice. This can lead to beri-beri, particularly prevalent in the Far East, which is characterized by ascending weakness in the legs and accompanying muscle tenderness, and can lead to widespread nerve irritability and congestive heart failure. This is due to a lack of thiamine, a vitamin that is essential for the mechanisms by which energy is released from foodstuffs.

Excess consumption of certain kinds of foods has been shown to be associated with the onset of conditions such as heart disease or diabetes — often exacerbating a genetic predisposition, and thus once more blurring the distinctions between different disease categories. Over consumption of alcohol, cigarette smoking, and taking other damaging drugs can all lead to disease conditions that can be classified as ‘self-inflicted’ (a category that can also include sexually transmitted diseases contracted during unprotected sex).

Food intolerances have been increasingly recognized in the latter part of the twentieth century, as some individuals show anaphylactic responses to particular allergens, such as nuts or dairy products.

Occupational diseases and the effects of pollution

Concern about the workplace as a source of disease has grown, particularly since World War II, as have the specialities of public and occupational health. Safety procedures, including the use of protective clothing, have been proposed to limit workers' exposure to dangerous chemicals or to hazardous practices. Working conditions, such as those for office workers using video display equipment, have received attention, and exposure to noise, bad ventilation, and poorly designed furniture have increasingly been recognized as playing a role in stress. Stress in turn is recognized as contributing to high blood pressure, heart disease, and stroke.

These concerns are not, however, entirely new. Lead poisoning amongst miners was recognized by Hippocrates (c.450 bce), but it was really the impact of the Industrial Revolution that focused the attention of reformers and some physicians on the impact of working conditions on health. A Leeds physician, Charles Thackrah, wrote The effects of arts, trades and professions on health and longevity (1832), which described the occupational hazards attached to numerous trades, including flock dressers, maltsters, coffee grinders, and corn-sillers. The effects of adverse and dangerous conditions on the ordinary working man became an issue for the growing number of trade or labour unions, and reform movements throughout the twentieth century campaigned for safer working conditions and adequate health care and compensation for those injured in the workplace.

Increasingly, however, environmental dangers have been recognized as having wider impact than just at the workplace. There can be pollution from accidental contamination and from large-scale industrial accidents. Disease and disasters can arise from cynical exploitation by manufacturers who ignore concerns for the welfare not only of their own workforce, but also of those living in the vicinity of their production facilities, such as the workers in the asbestos industry or the victims of the Bhopal explosion in India that killed 2000 people. Dispersal of pollutants, by air as after the Chernobyl disaster in Ukraine, or by river systems, can cause disease at vast distances from the original site of contamination.

Psychiatric diseases

Mental illness can refer to disorders in perception, understanding, emotion, and behaviour, and can range from the milder psychological disorders and psychosomatic illness to the severe psychosis. Psychiatric disorders have not always been seen to be the province of the medical profession: theories about demonic possession, for example, have led to religious remedies or persecution. For many centuries doctors had little to do with those classed as ‘insane’. The insane were incarcerated and contained, rather than treated. In the twentieth century increasing acknowledgement of the interplay of social, psychological, and physical factors in the causation of many psychiatric disorders, and the development of specific pharmacological therapies, led to improved care. Here again, the categories of disease classifications have become blurred, as faulty chemical processes in the brain and genetic defects have been shown to account for some manifestations of mental disease.

Degenerative diseases

Ironically, as infectious diseases were increasingly conquered during the twentieth century, degenerative diseases emerged in the West-ern world, primarily affecting the elderly. Degenerative processes can strike in particular organs or tissues, resulting in damaged joints, such as hips and knees; in weakened bones, as a result of osteoporosis; or as degeneration of the brain, causing severe mental deficits, such as dementia. It has been argued, most notably by the epidemiologist, Thomas McKeown (1912–88), that the main risks to life and good health have occurred in 3 distinct historical phases: accidents and injuries; infections; and finally degenerative diseases of longevity, which can include diseases such as Alzheimer's disease, Parkinson's diseases and some cancers.

One of the best known degenerative diseases is Alzheimer's, first described in 1906 by Alois Alzheimer, but then recognized as only a very rare brain disorder associated with cognitive dysfunction. This type of dementia is now the most common acquired progressive brain syndrome, although its cause remains unknown. Recent figures from the US have shown that Alzheimer's affects more than 4% of the over-60s population, whilst prevalence grows to 20% of the over-80s age group. The impact of chronic degenerative disease is felt not only by individuals and families, but also by social welfare and health care systems.


Cancer is caused by a breakdown in the normal processes of cell division and multiplication, resulting in uncontrolled cell growth producing a tumour. In the industrialized world, at the beginning of the twenty-first century, it is estimated that one-third of the population will develop cancer, with the probability currently increasing. This is partly because it is predominantly a disease of middle and old age, and as life expectancy has increased, so too has the incidence of cancer. Several cancers are known to have a genetic basis, and also the environmental impact of some pollutants, known as carcinogens, is becoming increasingly well understood.

Iatrogenic diseases

These diseases arise from medical treatment for another condition. Sometimes the problem may be due to recognized undesirable side-effects of therapeutic drugs, or to an unusual, idiosyncratic reaction to a medicament. A scheme of reporting adverse drug reactions, the so-called ‘yellow card scheme’ was introduced in Britain in the early 1970s, in an attempt to identify such reactions. Surgical procedures, when mishaps or infections result, can also inadvertently cause further disease.

E. M. Tansey


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McKeown, T. (1979). The role of medicine: dream, mirage or nemesis. Oxford University Press.
McKeown, T. (1988). The origins of human disease. Oxford University Press.

See also allergy; drug abuse; environmental toxicology; genetics, human; Islamic medicine; medicine; mind–body interaction; work and the body.

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Stedmans Online Medical Dictionary defines disease as an interruption, cessation, or disorder of body function, system, or organ; or a morbid entity characterized usually by at least two of these criteria: recognized etiologic agent(s), identifiable group of signs and symptoms, or consistent anatomic alterations. The International Classification of Disease, 9th Revision, Clinical Modification (ICD-9-CM) is one of the main texts used in the United States to identify, categorize, and diagnose disease. The Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV), is used to define and diagnosis mental disorders. While these sources are used in common medical practice, it is not completely clear in the philosophy of science what truly defines the diseases and disorders that these texts classify. Some have argued that there is not a simple definition of disease. Within the philosophy of medicine and bioethics, there is not only disagreement about what a disease is but whether or not disease can be defined or whether it is necessary to have a fixed definition in order to provide care.


A simplistic yet prevailing conception of what disease is can be viewed from the lens of naturalism or nominilism. Viewed from a naturalistic point of view, disease is a real thing that can be quantified, observed, or described using the language of natural science. To the naturalist, a disease can be discovered in nature, is not invented by social convention, and is not dependent on contextual circumstance. The true naturalist views disease as value free and objective. Disease from a naturalist point of view, according to the philosopher Christopher Boorse, causes interruptions in the ability to perform typical physiological functioning with at least typical efficiency (Kovacs 1998, p. 31). This point of view, however, has been critiqued because typical physiology and typical efficiency cannot be objectively described, nor are they value-free terms (i.e., what is meant by typical?).

The nominalist point, on the other hand, views disease not as something essential in nature but rather as a description of socially constructed conditions. As stated by Lester King in 1954, the point of view that disease is the aggregate of those conditions, which, judged by the prevailing culture, are deemed painful, or disabling, and which, at the same time, deviate from either the statistical norm or from some idealized status would fit within this nominalist point of view (King 1954, p. 197). Historically a purely naturalistic view of disease as a germ or lesion has given way to a view of disease that appeals more to a nominilist point of view. Ailments that fall within modern medical health care, such as depression or hypertension, challenge a naturalistic point of view because these conditions appeal to socially defined criteria by which one would be in need of professional care or qualify for some sort of intervention.


In discussing the concept of disease, attention has been brought to how terms such as disease, illness, or sickness relate. Oftentimes these concepts have been used interchangeably. However, philosophers argue that separating the concepts may be useful. Disease is distinguished from illness in that disease is the subject matter of the medical practitioner and scientific medicine. Illness, on the other hand, explains what the person is experiencing. Sickness is what is attributed by society to individuals who conceive of themselves as ill and whom medical professions identify as having a disease. Each of these concepts justifies action. Medical professionals are charged with identifying disease, discovering diseases, and treating persons with such conditions. Persons who are ill are charged with describing the subjective experience of their condition to others who may be able to help. Society is responsible for determining the rights and duties of a person who is ill and/or diseased. Thus conceptualizing disease as separate from illness and sickness can be useful in bringing into perspective the varying roles of the medical practitioner, the individual, and society when negative bodily conditions or states occur.

The concept of disease is also often discussed as it relates to health. That is, to understand what disease is, one must know what health is. The common language conception of health is simply the absence of disease or the negation of being at ease (i.e., dis-ease). A person who is healthy does not have a disease, and a person with a disease is not healthy. However, this simplistic model may not be applicable in all circumstances. For example, a person diagnosed with hypochondriasis certainly is suffering, but the individual does not have any general medical condition that can account for his or her feeling of illness. There are also instances when one feels healthy but may have a serious condition that places the individual at risk for a disease (e.g., a person with hypercholesterolemia or obesity may develop coronary artery disease).

The holistic approach extends the more simple approach to defining health not just as the absence of disease but as a state of complete physical, mental, and social well-being. The holistic model has been adopted and promoted by the World Health Organization. The holistic model would imply that one could meet the condition of not having a specific disease but still may not be healthy. Within the holistic model, eliminating disease from the body is not primary, but rather, health is primary. However, some have argued that a holistic program of health care with the goal of insuring complete physical, mental, and social well-being is not feasible; especially in developing countries, where there are limited resources available for the provision of care.

The model most familiar to Western medicine is the medical model of disease. The medical model suggests that disease is not just absence of health (as defined by the simplified model), but disease can be identified by some set of standard methods, such as a medical examination, laboratory tests, or correspondence with a set of symptoms. Thus within the medical model a person could potentially not have an identified pathophysiological disease but could still be labeled as having a disease as a result of having a set of symptoms and being deemed not healthy through the process of a medical examination.


Within modern medicine there are many controversies over what conditions can be properly defined as diseases. One such debate in the general medicine and public health has to do with whether or not obesity can be labeled a disease. George A. Bray, an internationally recognized researcher in the area of obesity and diabetes, has argued that obesity meets the criteria to be labeled a disease. However, other researchers have argued that caution should be taken when labeling obesity as a disease as it may not be appropriate to put it on par with other more serious life-threatening conditions. Those that argue obesity should not be considered a disease suggest that there are no real signs or symptoms of obesity apart from excess adiposity. However, this is circular because excess adiposity is the definition of obesity. Also, while obesity does cause impairment in functioning for some people, there are many people who are obese who have no diminished impairment in functioning. Those who argue that obesity is a disease equate it with other diseases, such as depression. Bray states that obesity involves deranged neural circuitry responding inappropriately to a toxic environment (Bray 2004, p. 34).

Another long-standing debate in the medical discipline of psychiatry is whether or not certain psychological conditions can be labeled a disease. A mainstream view of modern practice in psychiatry is that certain psychological conditions rise to the level of an illness when there is a clinically relevant disruption in functioning and distress. The DMS-IV distinguishes a mental pathological condition from a milder form by establishing clinically significant criterion. As stated in the DSM-IV, the condition must cause clinically significant distress or impairment in social, occupational, or other important areas of functioning (DSM-IV 1994, p. 7). The determination of significance is a clinical judgment made through the process of a clinical interview with the patient and sometimes with third parties, such as a patients family. Further, a mental disorder is often distinguished from a condition that arises as a direct physiological consequence of a general medical condition. For example, disorientation or hallucinations due to a brain tumor or stroke would not be considered a psychiatric condition.

Notably the explanation of aberrant behaviors or mental conditions has changed over the centuries. In past centuries aberrant behaviors and mental disorders were explained as the result of spirits or sins. With the birth of psychoanalysis, mental conditions were explained as primarily resulting from poor child rearing or the inability of an individual to meet developmental milestones marking social and moral development. However, early twenty-first-century psychiatric practice tends to explain many psychiatric conditions as the result of disruptions in neural circuitry in the brain resulting from a combination of genetic and environmental determinates.

This change in perception of psychiatric conditions is argued to be due to an increasing scientific knowledge about potential causes and treatments. However, the psychiatrist Thomas Szasz has been a prominent critic of this traditional point of view. Szasz argues that mental disorders, as mainstream psychiatry has conceptualized, are not diseases of the brain and that it is inappropriate to call abnormal behaviors and psychological states diseases. A crux of difference between these two points of view has to do with the way disease is definedthat is, as a lesion of the body or as a social construction or metaphor.


Regardless of how disease is defined, it is widely recognized that the spread of disease and the preponderance of health are linked to social factors. For instance, density and frequency of contact among individuals can influence disease outbreak. Dense social contact in urban environments may lead to a rapid spread of certain infectious diseases. Understanding the social networks and dynamics of these environments is a key strategy for developing targeted vaccinations and treatments.

Disease and health are also influenced by social and economic conditions in society. For example, in the early twenty-first century in the United States, Type 2 diabetes mellitus is more common among African American men than their Caucasian counterparts. However, a 2007 study by Margaret Humphreys and colleagues found rates of diabetes among African American men living circa 1900 to be much lower than Caucasian men at that time. Studies looking at coronary heart disease patterns have also reported prevalence shifts whereby the risk of the disease was historically more prevalent in higher socioeconomic classes and now is more prevalent in lower socioeconomic classes (Kunst et al., 1999; Marmot, Adelstein, Robinson, and Rose, 1978; Rose and Marmot, 1981). These studies highlight the fact that disease patterns as well as the social distribution of risk factors for disease can vary by type of disease, time period, and geographic region.

As disease and health are viewed as socially determined, the search for social conditions that gives rise to diseases has become a growing part of medical and public health science. Medical practice in the past centuries was focused primarily on identifying pathophysiological and biological roots for disease and had largely ignored the social contributions to disease. Correspondingly treatments and interventions for disease management have been one-to-one efforts. However, a growing awareness that societal-level phenomena play a large role in health and disease has prompted the medical community to explore some of the broader social and economic forces that influence disease and risk. As such the approach to disease management is also shifting from primarily individual-level one-to-one efforts to include environmental and policy-level interventions designed to address health.

Finding a clear definition of disease and health is not purely a philosophical matter. Conditions that carry the label of disease have practical and political implications. Society responds by directing resources, and individuals with a certain disease are relinquished from certain social responsibilities. However, what counts as disease is often difficult to determine. In some cases it might appear that a certain condition has pathophysiological roots and causes (e.g., germ or lesion) that can be discovered and treated. However, it may be discovered that there are broader social and economic conditions that allow for certain pathophysiological conditions to arise. What then is the disease? Is it the germ or the social condition? The answer that society provides becomes one of the defining features by which health care resources are allocated.

SEE ALSO Alzheimers Disease; Dementia; Depression, Psychological; Ethno-epidemiological Methodology; Functionings; Human Rights; Hypertension; Madness; Malnutrition; Medicine; Mental Illness; Obesity; Poverty; Psychoanalytic Theory; Public Health; World Health Organization


American Psychiatric Association. 1994. Diagnostic and Statistical Manual of Mental Disorders. 4th ed. Washington, DC: Author.

Boorse, Christopher. 1977. Health as a Theoretical Concept. Philosophy of Science 44: 542573.

Bray, George A. 2004. Obesity Is a Chronic, Relapsing Neurochemical Disease. International Journal of Obesity and Related Metabolic Disorders 28 (1): 3438.

Bray, George A. 2006. Obesity: The Disease. Journal of Medicinal Chemistry 49 (14): 40014007.

Centers for Disease Control and Prevention. 2006. National Diabetes Surveillance System.

Eubank, Stephen, Hansan Guclu, V. S. Anil Kumar, et al. 2004. Modeling Disease Outbreaks in Realistic Urban Social Networks. Nature 429 (6988): 180184.

Heshka, Stanley, and David B Allison. 2001. Is Obesity a Disease? International Journal of Obesity and Related Metabolic Disorders 25 (10): 14011404.

Hofmann, Bjorn. 2001. Complexity of the Concept of Disease as Shown through Rival Theoretical Frameworks. Theoretical Medicine and Bioethics 22 (3): 211236.

Hofmann, Bjorn. 2002. On the Triad: Disease, Illness, and Sickness. Journal of Medicine and Philosophy 27 (6): 651673.

Hofmann, Bjorn. 2005. Simplified Models of the Relationship between Health and Disease. Theoretical Medicine and Bioethics 26 (5): 355377.

Hofmann, Bjorn M., and Harald M. Eriksen. 2001. The Concept of Disease: Ethical Challenges and Relevance to Dentistry and Dental Education. European Journal of Dental Education 5 (1): 211.

Humphreys, Margaret, Philip Costanzo, Kerry L. Haynie, et al. Racial Disparities in Diabetes a Century Ago: Evidence from the Pension Files of the U.S. Civil War Veterans. Social Science and Medicine 64 (8): 17661775.

King, Lester S. 1954. What Is Disease? Philosophy of Science 21: 193203.

Kottow, Michael H. 2002. The Rationale of Value-Laden Medicine. Journal of Evaluation in Clinical Practice 8 (1): 7784.

Kovacs, Jozsef. 1998. The Concept of Health and Disease. Medical Health Care and Philosophy 1 (1): 3139.

Kunst, Anton E., Feikje Groenhof, Otto Andersen, et al. 1999. Occupational Class and Ischemic Heart Disease Mortality in the United States and 11 European Countries. American Journal of Public Health 89 (1): 4753.

Rose, Geoffrey, and Michael Marmot. 1981. Social Class and Coronary Heart Disease. British Heart Journal 45 (1): 1319.

Szasz, Thomas. 1974. The Myth of Mental Illness: Foundations of a Theory of Personal Conduct. New York: Harper and Row.

Szasz, Thomas. 1998. Parity for Mental Illness, Disparity for the Mental Patient. Lancet 352 (9135): 12131215.

Szasz, Thomas. 1998. What Counts as Disease? Rationales and Rationalizations for Treatment. Forsch Komplementarmed 5 (S1): 4046.

Twaddle, Andrew. 1994. Disease, Illness, and Sickness Revisited. In Disease, Illness, and Sickness: Three Central Concepts in the Theory of Health, eds. Andrew Twaddle and L. Nordenfelt, vol. 18, pp. 118. Linkoping, Sweden: Studies on Health and Society.

Vagero, Denny, and Mall Leinsalu. 2005. Health Inequalities and Social Dynamics in Europe. British Medical Journal 331 (7510): 186187.

World Health Organization. 1992. Basic Documents. Geneva: World Health Organization.

Bernard F. Fuemmeler

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Disease can be defined as any change in body processes that impairs its normal ability to function. The human body has certain basic requirements that must be met if it is to function normally. These requirements include the proper amount of oxygen, acidity, salinity, and other conditions. These conditions must all be maintained within a very narrow range. A deviation from that range can cause disease to develop.

Most diseases can be classified into one of three major categories: infectious diseases; noninfectious diseases; and diseases for which no cause has yet been identified. At one time, a number of conditions were also classified as genetic diseases. This category includes conditions such as sickle-cell anemia, phenylketonuria, Tay-Sachs disease, cystic fibrosis, and galactosemia. These conditions are now more appropriately known as genetic disorders.

Infectious diseases

At one time, humans were totally mystified as to the causes of common diseases such as typhoid, typhus, pneumonia, mumps, yellow fever, pneumonia, smallpox, rabies, syphilis, gonorrhea, tuberculosis, and rheumatic fever. Explanations ranged from punishment by God for evil deeds to acts of magicians or witches to an unbalance in the composition of the blood.

During the eighteenth century, the true nature of such diseases was finally discovered. Largely due to the work of the French chemist Louis Pasteur (18221895) and the German bacteriologist Robert Koch (18431910), scientists learned that infectious diseases were caused by organisms that entered the human body and upset its normal healthy state. In most cases, these organisms were too small to be seen with the unaided eye: bacteria, viruses, and fungi, for example. In other cases, they were caused by various types of worms. Diseases of the latter type are usually called parasitic diseases because the worms live off the human body as parasites.

The human body includes a number of devices to protect itself from infectious diseases. The first in line of these devices is skin. Skin can be thought of as a protective envelope surrounding the body. That envelope generally is able to prevent disease-causing organisms (germs) from entering the body.

One way in which disease can develop is for a break to occur in the skin, as in a cut or scrape. Germs that would normally be prevented from

entering the body are able to invade the bloodstream through such openings. At that point, the body puts into action a second line of defense: the immune system. The immune system is a complicated collection of chemical reactions that release compounds that attack and destroy invading organisms. Without an immune system, the human body would become ill nearly every time there was a cut in the skin.

In some instances, the immune system is unable to react adequately to an invasion of germs. In such cases, disease develops.

The spread of infectious disease. One characteristic of infectious diseases is that they are easily transmitted from one person to another. For example, a person who has contracted typhus can easily pass that disease to a second person simply by coming into contact with that person. Germs travel from the carrier of the disease to the uninfected person.

Disease can be spread by many methods other than direct contact, such as through water, food, air, and blood. Waterborne transmission occurs through contaminated water, a common means by which cholera, waterborne shigellosis, and leptospirosis are spread. Foodborne poisoning in the form of bacterial contamination may occur when food is improperly cooked, left unrefrigerated, or prepared by an infected food handler.

Diseases such as measles and tuberculosis can be transmitted through the air. Any time an infected person coughs or sneezes, infectious organisms can travel more than 3 feet (0.9 meter) to an uninfected person. Fungal infections such as histoplasmosis, coccidioidomycosis, and blastomycosis can also be spread by airborne transmission as their spores are transported on dust particles.

Vectors are animals that carry germs from one person to another. The most common vectors are insects. These vectors may spread a disease either by mechanical or biological transmission. An example of mechanical transmission occurs when flies transfer the germs for typhoid fever from the feces (stool) of infected people to food eaten by healthy people. Biological transmission occurs when an insect bites a person and takes in infected blood. Once inside the insect, the disease-causing organisms may reproduce in the gut, increasing the number of parasites that can be transmitted to the next person. The disease malaria is spread by the Anopheles mosquito vector.

Epidemics. Diseases sometimes spread widely and rapidly through a population. Such events are known as epidemics. One of the best-known epidemics in human history was the Black Death that struck Europe in the mid-fourteenth century. Caused by the microorganism Pasteurella pestis, the Black Death is also known as the bubonic plague, or simply, plague. Plague is transmitted when fleas carried by squirrels and rats bite humans and transfer the P. pestis from one person to another.

Once it reached Europe from Asia in about 1350, the plague was virtually unstoppable. In some areas, whole towns were destroyed as people either died or moved away trying to avoid the disease. Over an eight-year period, an estimated 25 million people died of the disease.

Other examples of epidemics include the worldwide spread of cholera during the mid-nineteenth century, the influenza epidemic in the United States in the early twentieth century, and the HIV (human immunodeficiency virus) epidemic in the United States beginning in the early 1980s.

Protection against infectious diseases. When scientists learned the cause of infectious diseases, they also developed the ability to prevent and cure such diseases. For example, people can now be vaccinated as a protection against many types of infectious disease. A vaccine is a material that can be injected into a person to ward off attacks by certain disease-causing organisms. The material may consist of very weak concentrations of the organism itself or of dead organisms. The presence of these organisms in the bloodstream stimulates the body's immune system to start producing chemicals that will fight off the disease if and when it actually enters the body.

In addition, scientists have discovered and invented a host of substances that will fight the germs that cause infectious diseases. The class of drugs known as antibiotics, for example, can be used to aid the body's natural immune system in combatting disease-causing organisms that have entered the body.

Childhood diseases. Chicken pox, measles, and mumps are all common childhood diseases. The term childhood disease is a bit misleading, however, since any one of these diseases can be contracted by a person at any age. The term developed simply because the diseases are much more common among young children than they are among adults.

The diseases named above are all infectious, caused by a virus. They are generally spread by direct contact between an infected and a noninfected person, and since young children are often in direct contact with each otheron the playground, riding a school bus, or in a classroomthey are especially susceptible to such diseases.

All three viral diseases have a somewhat similar pattern. There is a period of incubation, during which the virus reproduces within a person's body. Obvious symptoms then begin to appear: a rash in the case of chicken pox and measles and inflamed and swollen glands in the case of mumps. All three diseases normally disappear after a period of time, generally without leaving any long-term effects.

Most children can now be protected against childhood diseases by means of a regular program of immunization (vaccinations). There are, as yet, however, few if any treatments for the diseases themselves.

Noninfectious diseases

Vaccinations and drugs have been so successful in treating infectious diseases that they are no longer the massive threat to human health that they once were. Today, the greatest threat to human health are noninfectious diseases such as heart disease, cancer, and diseases of the circulatory system. In some cases, the nature of these diseases is well understood, and medical science is making good progress in combatting

them. For example, it is known that a stroke occurs when arteries in the brain become constricted or clogged and are unable to permit the normal flow of blood. The brain is deprived of blood, and cells begin to die, causing loss of muscular control, paralysis, and, eventually, death.


Your town is in a state of panic. Dozens of people have become ill in the past month with a disease that no one can recognize. You and your neighbors are worried that you too will become ill with the disease. To whom can you turn for help?

This puzzle calls for the work of an epidemiologist. An epidemiologist is a scientist who studies the cause and spread of disease. The epidemiologist uses a number of sophisticated techniques in his or her work. One of these techniques is sometimes called source and spread. Interviews are held with people who are ill to find out those with whom they have recently come into contact. The goal is to find out from whom the person got the disease and to whom it might have been passed on. This pattern of disease spread is sometimes called a web of causation.

Epidemiologists also try to track down the agent that caused the disease: a bacterium, virus, fungus, or other organism. They then try to determine how that organism has been transmitted from one person to another. By identifying the specific factors involved in an epidemic, it is sometimes possible to determine preventive actions that can be taken to reduce the occurrence of a disease. For example, it may be that everyone who has come down with the disease in your town has been swimming in the local lake. The disease can be prevented from spreading, then, by warning people not to swim in that lake.

The techniques of epidemiology have also been used to deal with noninfectious diseases. For example, some epidemiologists have argued that gun-related accidents have many of the characteristics of an infectious diseases. They say that people who are injured by guns can be studied in much the same way as people who become ill because of a disease-causing organism. This idea is still relatively new, however, and has yet to prove its worth in dealing with such problems.

Another noninfectious disease is cancer. The term cancer refers to any condition in which cells in a person's body begin to grow in a rapid and uncontrolled way. The causes of such growth are probably many and varied. For example, certain types of chemicals (known as carcinogens) can cause cancer. Certain kinds of tars, dyes, and organic compounds are known to be responsible for various forms of cancers. The largest single fatal form of cancer, lung cancer, is caused by chemicals found in tobacco smoke. Exposure to various forms of radiation are also known to cause cancer. People who are exposed to long periods of sunshine are at high risk for the development of various forms of skin cancer, the most dangerous of which is malignant melanoma. Some scientists also believe that some forms of cancer may be caused by viruses (which would make them an infectious disease).

[See also Ebola virus; Genetic disorders; Legionnaires' disease; Plague ]

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disease, impairment of the normal state or functioning of the body as a whole or of any of its parts. Some diseases are acute, producing severe symptoms that terminate after a short time, e.g., pneumonia; others are chronic disorders, e.g., arthritis, that last a long time; and still others return periodically and are termed recurrent, e.g., malaria. One of the most common bases for classifying disease is according to cause. External factors that produce disease are infectious agents, including both microscopic organisms (bacteria, viruses, and protozoans) and macroscopic ones (fungi and various parasitic worms). Only infectious diseases can be transmitted—by humans, certain animals and insects, and infected objects and substances (see communicable diseases). Other external agents that can cause disease are chemical and physical agents (drugs, poisons, radiation), which can be encountered in specific work situations, deficiency of nutrients in the environment, and physical injury. Diseases that arise from internal (endogenous) causes include hereditary abnormalities (disorders inherited from one or both parents), congenital diseases (disturbances in the development of a normal embryo), allergies (hypersensitive reactions to substances in the environment), endocrine disorders (generally either overfunctioning or underfunctioning of an endocrine gland), circulatory disorders (diseases of the heart and blood vessels), and neoplasms, or tumors (masses of abnormally proliferating cells). Degenerative diseases occur as a result of the natural aging of the body tissues. Finally, a wide range of diseases are attributed to, or at least influenced by, emotional disturbances. Psychoses and neuroses result in disturbed behavior; the so-called psychosomatic diseases (certain kinds of colitis, many forms of headaches) are thought to be brought about by emotional stress. Most diseases occur as a result of a combination of both internal and external conditions, i.e., an interaction between the body and the environment. Thus a person may be hereditarily predisposed to tuberculosis, although the tubercule bacillus (the infectious agent) must be present for the disease to occur. In ancient times disease was ascribed to supernatural, spiritual, and humoral factors. The discovery by Louis Pasteur and others of the role played by microorganisms in infection and the study of cellular pathology by Rudolf Virchow in the 19th cent. were of the utmost importance in establishing the true nature of disease.

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188. Disease

  1. AIDS mysterious new disease, incurable and usually fatal. [U.S. Hist.: WB, A:153]
  2. Black Death killed at least one third of Europes population (13481349). [Eur. Hist.: Bishop, 379382]
  3. bubonic plague ravages Oran, Algeria, where Dr. Rieux perseveres in his humanitarian endeavors. [Fr. Lit.: The Plague ]
  4. Cancer Ward, The novel set in cancer ward of a Russian hospital. [Russ. Lit.: The Cancer Ward in Weiss, 64]
  5. Decameron, The tales told by young people taking refuge from the black death ravaging Florence. [Ital. Lit.: Magill II, 231]
  6. Fiacre, St. intercession sought by sick. [Christian Hagiog.: Attwater, 130]
  7. influenza epidemic caused 500,000 deaths in U.S. alone (19181919). [Am. Hist.: Van Doren, 403]
  8. Joram suffered for abandoning Gods way. [O.T.: II Chronicles 21:15, 19]
  9. Journal of the Plague Year Defoes famous account of bubonic plague in England in 1665. [Br. Lit.: Benét, 529]
  10. Lazarus leper brought back to life by Christ. [N.T.: John 11:144]
  11. Legionnaires disease 28 American Legion conventioneers die of flu-like disease in Philadelphia (1976). [Am. Hist.: Facts (1976), 573, 656]
  12. Molokai Hawaiian island; site of government leper colony. [Am. Hist.: NCE, 1807]
  13. Naaman leprous Syrian commander healed by Elisha. [O.T.: II Kings 5]
  14. red death, the pestilence, embodied in a masque, fatally penetrates Prince Prosperos abbey. [Am. Lit.: Poe The Masque of the Red Death ]
  15. Rock, St. legendary healer of plague victims. [Christian Hagiog.: Attwater, 299]
  16. Sennacherib, army of besieging Jerusalem, Assyrian force must withdraw after an outbreak of plague. [O. T.: II Kings 19:35; Br. Lit.: Byron The Destruction of Sennacherib in Benét, 266]
  17. seven plagues, the visited upon the earth to signify Gods wrath. [N.T.: Revelation]
  18. St. Anthonys Fire horrific 11th-century plague. [Eur. Hist.: Brewer Note-Book, 34]
  19. Syphilis Fracastoros epic concerning Syphilis, mythical first victim. [Ital. Lit.: RHD, 1443; Plumb, 342]
  20. ten plagues, the inflicted upon Egypt when Pharaoh refuses to let the Israelites emigrate. [O.T.: Exodus 7-12]
  21. Typhoid Mary (Mary Mallon, 18701938) unwitting carrier of typhus; suffered 23-year quarantine. [Am. Hist.: Van Doren, 354]

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Disease (or "lack of ease") is any damage or injury that impairs an organism's function. Diseases (sometimes called deviations from the norm) can be classified in numerous ways. Generally, an acute disease comes on quickly and lasts for only a relatively short time. A chronic disease usually begins slowly and lasts for a longer time.

Diseases can also be classified according to type. Common disease types or categories include: infectious, genetic (hereditary), psychiatric, deficiency, degenerative, congenital (whether genetic or not), neurological, cardiovascular, metabolic, chemical, and occupational.

Infectious or microbial diseases (the pathogenic diseases) are often classified by their causative agents: bacteria, fungi, protozoans, viruses, or prions. Progressive diseases, particularly those caused by microbes, have several clinical stages: infection, incubation, acute, decline, and convalescent. "Prodromal" refers to the initial stages when perhaps only one or two early characteristics of the disease can be observed. Communicable diseases are transmitted either directly or indirectly (via carriers or vectors ) from one organism to another. Contagious diseases are rapidly transmitted infectious diseases. Malignant diseases usually progress quite rapidly and are potentially life threatening.

In contrast to disease (the deviation itself), "illness" is feeling of being sick, or suffering some effect of the disease. Many people have hypertension (the disease), for example, without feeling any illness until it is so far advanced that is causes a stroke or kidney failure. This is why hypertension is sometimes called "the silent killer."

see also Autoimmune Disease; Bacterial Diseases; Cardiovascular Diseases; Genetic Diseases; History of Medicine; Homeostasis; Neurologic Diseases; Parasitic Diseases; Psychiatric Disorders, Biology of; Sexually Transmitted Diseases; Viral Diseases

Roberta M. Meehan


Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Thomas, Clayton L., ed. Taber's Cyclopedic Medical Dictionary, 18th ed., Philadelphia, PA: F. A. Davis Company, 1997.

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disease Any departure from health, with impaired functioning of the body. Disease may be acute, severe symptoms for a short time; chronic, lasting a long time; or recurrent, returning periodically. There are many types and causes of disease: infectious, caused by harmful bacteria or viruses; hereditary and metabolic; growth and development; immune system diseases; neoplastic (tumour-producing); nutritional; deficiency; endocrine system diseases; or diseases due to environmental agents, such as lead poisoning. Treatment depends on the cause and course of the disease. It may be symptomatic (relieving symptoms, but not necessarily combating a cause) or specific (attempting to cure an underlying cause). Disease prevention includes eradication of harmful organisms, vaccines, public health measures and routine medical checks.

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"disease." World Encyclopedia. . 10 Dec. 2017 <>.

"disease." World Encyclopedia. . (December 10, 2017).

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dis·ease / diˈzēz/ • n. a disorder of structure or function in a human, animal, or plant, esp. one that produces specific signs or symptoms or that affects a specific location and is not simply a direct result of physical injury: bacterial meningitis is a rare disease | a possible cause of heart disease. ∎ fig. a particular quality, habit, or disposition regarded as adversely affecting a person or group of people: departmental administration has often led to the dread disease of departmentalitis.

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"disease." The Oxford Pocket Dictionary of Current English. . 10 Dec. 2017 <>.

"disease." The Oxford Pocket Dictionary of Current English. . (December 10, 2017).

"disease." The Oxford Pocket Dictionary of Current English. . Retrieved December 10, 2017 from


disease A condition in which the normal function of some part of the body (cells, tissues, or organs) is disturbed. A variety of microorganisms and environmental agents are capable of causing disease. The functional disturbances are often accompanied by structural changes in tissue.

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"disease." A Dictionary of Biology. . 10 Dec. 2017 <>.

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disease (di-zeez) n. a disorder with a specific cause and recognizable signs and symptoms; any bodily abnormality or failure to function properly, except that resulting directly from physical injury (the latter, however, may open the way for disease).

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"disease." A Dictionary of Nursing. . 10 Dec. 2017 <>.

"disease." A Dictionary of Nursing. . (December 10, 2017).

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disease †uneasiness, discomfort; morbid physical condition. XIV. — AN. des-, disease, OF. desaise, f. des- DIS- 2, 6 + aise EASE.

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"disease." The Concise Oxford Dictionary of English Etymology. . 10 Dec. 2017 <>.

"disease." The Concise Oxford Dictionary of English Etymology. . (December 10, 2017).

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"disease." A Dictionary of Sociology. . 10 Dec. 2017 <>.

"disease." A Dictionary of Sociology. . (December 10, 2017).

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