Parasitic Disease

parasitic disease, in humans, any illness that is caused by a parasite, an organism that lives in or on another organism (known as the host). Parasites typically benefit from such relationships, often at the expense of the host organisms. Parasites of humans include protozoans, helminths, and ectoparasites (organisms that live on the external surface of a host). They are responsible for many diseases and are transmitted to their hosts most often through the ingestion of contaminated food or water or through the bite of an arthropod (e.g., a fly or tick), which can act as an intermediate host and as a vector. (For information on parasitic diseases in animals, see animal disease: Survey of animal diseases. For information on parasitic diseases in plants, see plant disease: Classification of plant diseases by causal agent.)

Disease-causing parasites have long affected human populations. Calcified helminth eggs, for example, have been recovered from Egyptian mummies dated to about 1200 bce, and written records indicate that ancient Greek and Roman physicians treated patients with various nematode infections, including tapeworm. From the 17th to the 20th centuries, with the discovery and classification of numerous parasites, came the realization of the global burden of parasites. Indeed, more than 3 billion people worldwide are infected by intestinal parasites or protozoans, and parasitic diseases are among the leading causes of deaths in humans globally. Epidemiological studies indicate that multiple factors influence a person’s risk of infection and the spread of parasitic disease, including parasite pathogenicity, host health, environment, and social conditions.