This is an elementary textbook for the use of students and practitioners dealing with protozoan and helminthic parasites and the diseases associated with these infections. The author expresses the view that these maladies are no longer limited to tropical countries, since modern transportation has disseminated them, "with few exceptions, to practically all latitudes inhabited by man." It is doubtless true that acceleration of the speed of transportation, increases in numbers of travelers, and the greater movement of men in and out of the tropics all combine to mingle infections from a greater variety of sources. But the truly tropical diseases, such as trypanosomiasis, yellow fever and Guatemalan onchocerciasis, are still limited to the tropics and near tropics by their dependence on insect vectors with a tropical distribution. Absence of sanitation and climatic and dietary factors also combine to make certain diseases more disastrous in the tropics than in temperate regions.