Entamoeba histolytica trophozoite in the stool. Entamoeba is a facultative anaerobic protist that forms trophozoites as well as cysts.
The life cycle of Entamoeba histolytica involves trophozoites that live in the host's large intestine and cysts that are passed in the host's feces. Humans are infected by ingesting cysts, most often via food or water contaminated with human fecal material. The trophozoites can destroy the tissues that line the host's large intestine, so of the amoeba infecting the human gastrointestinal tract, E. histolytica is potentially the most pathogenic. In most infected humans the symptoms of "amoebiasis" are intermittent and mild (various gastrointestinal upsets, including colitis and diarrhea). In more severe cases the gastrointestinal tract hemorrhages, resulting in dysentery. In some cases the trophozoites will enter the circulatory system and infect other organs, most often the liver (hepatic amoebiasis), or they may penetrate the gastrointestnal tract resulting in acute peritonitis; such cases are often fatal. As with most of the amoeba, infections of E. histolytica are often diganosed by demonstrating cysts or trophozoites in a stool sample.