The subject matter of this book is based primarily on the material in parasitology included, with information on other infectious agents, in the course of Infectious Diseases given in the School of Medicine, University of California at Los Angeles. The method of presentation is according to organs or organ systems of the patient. Following two introductory chapters, the first of which is in a sense a continuation of the Preface and the second a succinct but comprehensive consideration of Parasites, Parasitism and Host Relations, the third chapter provides instruction for examination of the feces for discovery of intestinal protozoa. Chapter four presents descriptions of the six species of intestinal amebae reported from man, the intestinal flagellates, the two species of human Isospora, and Balantidium coli, followed by a brief description of the protozoa of the mouth. Chapter five considers in some detail the Intestinal Helminths, namely the trematodes, cestodes and roundworms.