Entamoeba histolytica is the usual cause of intestinal amebiasis, and has the microscopic features described in the question stem. A paicularly helpful (but not always present) feature of this organism is the presence of ingested red blood cells within the amoebae. These amoebae cause flask-shaped ulceration of the intestinal mucosa and submucosa, with a paicular propensity for involving the cecum and ascending colon. The disease manifestations range from none (asymptomatic carriers) to mild chronic diarrhea, to severe, purging dysentery. In symptomatic cases, the liver may develop destructive amoebic liver abscesses that tend to become secondarily (and potentially life-threateningly) infected by bacteria. Acanthamoeba is a free-living amoebae that can cause amoebic meningocephalitis. Balantidium coli is a large ciliated intestinal parasite that can occasionally cause colonic disease resembling that caused by Entamoeba histolytica. Giardia lamblia is a small intestinal protozoa with a distinctive pear-shaped morphology that appears to have a "face." Ref: Ray C.G., Ryan K.J. (2010). Chapter 51. Rhizopods. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.