Infection: Infectious Diseases

Infectious diseases are a group of illnesses caused by various microbes (organisms too small to be seen without a microscope), which may be bacteria, viruses, fungi, protozoa, parasites, or abnormal proteins called prions. Infectious diseases are contagious. They may spread by physical contact with other infected persons or contaminated objects; by contact with body fluids; by eating or drinking contaminated food or water; by airborne droplets; or by vectors—insects or other animals that transmit disease organisms to humans.

Infectious diseases can spread over a wide geographical area, causing epidemics or pandemics. Some historical pandemics that caused widespread loss of life include the Black Death in the fourteenth century; the smallpox epidemics in Europe and the Americas in the eighteenth century; and the influenza pandemic of 1918–1919.

The microbes that cause infectious diseases are defined as either primary or opportunistic depending on whether they cause disease in people with normal immune systems or whether they can cause disease only in people with weakened immune systems, also called opportunistic infections.

Infectious diseases are still a major cause of death worldwide. According to the World Health Organization (WHO), about 15 million people die each year of infectious diseases—about 26 percent of all

deaths. The major killers as of 2008 were pneumonia and influenza, diarrheal diseases, AIDS, and malaria.

SEE ALSO AIDS; Anthrax; Avian influenza; Bronchitis; Chickenpox; Chlamydia; Cold sore; Common cold; Conjunctivitis; Creutzfeldt-Jakob disease; Ear infection; Ebola and Marburg hemorrhagic fevers; Encephalitis; Food poisoning; Genital herpes; Gonorrhea; Hantavirus infection; Hepatitis A; Hepatitis B; Hepatitis C; HPV infection; Infectious mononucleosis; Influenza; Lyme disease; Malaria; Measles; Meningitis; Necrotizing fasciitis; Periodontal disease; Plague; Pneumonia; Polio; Rabies; Rheumatic fever; Rubella; Scarlet fever; Severe acute respiratory syndrome; Smallpox; Sore throat; Staph infection; Strep throat; Syphilis; Tetanus; Tooth decay; Toxic shock syndrome; Toxoplasmosis; Tuberculosis; Ulcers; Urinary tract infection; Warts; West Nile virus infection; Whooping cough