Synopsis Infections of humans caused by parasites number in the billions and range from relatively innocuous to fatal. The diseases caused by these parasites constitute major human health problems throughout the world. (For example, approximately 30 per cent of the world's population is infected with the nematode Ascaris lumbricoides.) The incidence of many parasitic diseases (e.g., schistosomiasis, malaria) have increased rather than decreased in recent years. Other parasitic illness have increased in importance as a result of the AIDS epidemic (e.g., cryptosporidiosis, Pneumocystis carinii pneumonia, and strongyloidiasis). The migration of parasite-infected people, including refugees, from areas with high prevalence rates of parasitic infection also has added to the health problems of certain countries.