A number of species of ameba inhabit the gastrointestinal tract of primates. Fortunately most are non-pathogenic and exist as harmless lumen-dwelling commensals. One species, Entamoeba histolytical, although often an innocuous lumen-dweller, can invade tissue and produce severe and even fatal disease in both nonhuman primates and man. Amebiasis, or amebic dysentery, produces primary lesions in the colon and cecum, but metastatic lesions may appear in the liver and more rarely in the lungs and brain.