Parasitic protozoa are major causes of disease, particularly in developing countries. They do not secrete exotoxins but are able, according to species, to colonize the gastrointestinal tract, invade the circulating blood, or develop within cells. Among the important parasites of humans are the many types of plasmodia, amebae, cryptosporidia, trypanosomatids, and others. Only some are etiologic agents of lymphadenitides. Toxoplasma, Leishmania, and Filaria are common in various parts of the world and may cause specific lymphadenitides.