Infection: Parasitic Diseases

A parasite is an organism that lives in close association with another plant or animal of a different species. The organism on which the parasite lives or feeds is called the host. Animal parasites cause harm to their hosts in many different ways, by irritating or damaging the skin or hair, robbing the host of nourishment, causing blood loss, or carrying bacteria and viruses that cause disease.

Parasitic diseases are a group of illnesses caused by organisms that live either inside the body, like protozoa (one-celled organisms) or helminths (worms); or outside the body, like lice and fleas. Some parasitic diseases that affect humans are carried by insects or other animals. These are called vectors. For example, rat fleas are vectors of plague. Infected fleas

live as parasites on the rats and pass on the plague bacteria to humans when the rats die and the fleas need a new host. Similarly, cats are vectors of toxoplasmosis. This disease is caused by a protozoan (one-celled organism) that lives inside cats but can infect humans who touch cat feces containing the organism.

Parasitic diseases are a challenge to public health doctors because there are no vaccines available to prevent these illnesses. Controlling and treating these diseases depends almost entirely on medications, good sanitation, and careful attention to personal cleanliness and safe food handling practices.

SEE ALSO AIDS; Anemias; Lice infestation; Lyme disease; Malaria; Plague; Toxoplasmosis