This chapter analyzes intestinal protozoal infection that produces substantial morbidity and mortality in people of all ages, particularly in tropical and subtropical parts of the world. It cites amebiasis, giardiasis, cryptosporidiosis, and infections associated with AIDS as important problems for health in many parts of the world. It also examines intestinal protozoa that produce important human infections and different impacts in patients with AIDS. The chapter discusses Entamoeba histolytica, which causes dysentery, chronic colonic amebiasis, and hepatic amebiasis. It also reviews amebic dysentery, a syndrome of bloody diarrhea that is caused by invasion of the colonic wall by trophozoites of E. histolytica.