A minute protozoan organism consisting of a single cell, in which a nucleus is surrounded by protoplasm that changes its shape as the protozoon progresses or absorbs nourishment. Several varieties are found under different conditions within the human body. One variety, Entamoeba coli, may be found in the large intestine without any associated disease; another, Entamoeba gingivalis, may be found in the sockets of the teeth and is associated with PYORRHOEA. Entamoeba histolytica is the causative organism of amoebic dysentery (see DYSENTERY); rarely, Acanthamoeba and Naegleria fowleri may cause amoebic MENINGOENCEPHALITIS.