Any protozoan of the genus Entamoeba. Most are parasites in the intestines of vertebrates, including humans. E. histolytica causes human amebic dysentery. Infection of the large intestine with E. histolytica often causes no symptoms; however, diarrhea, abdominal pain, and fever may result from invasion and ulceration of intestinal walls. In the worst cases, the liver, lungs, brain, and spleen may be affected. Cysts of E. histolytica are transmitted through food and water, often by fly and cockroach droppings. E. gingivalis, found especially in unhealthy mouths, has not been shown to cause disease.