Balantidium, genus of ovoid protozoans of the holotrichous order Trichostomatida. Uniformly covered with longitudinal rows of minute, hairlike projections (cilia), Balantidium exists as a parasite in the intestines of pigs, apes, and other animals. The species B. coli can, in rare cases, infect humans and cause balantidiosis (balantidial dysentery), a relatively severe disease causing formation of intestinal ulcers. Balantidium, which feeds on red blood cells, cell fragments, and cell debris, is transmitted to new hosts by rounded cysts passed in excrement and subsequently ingested by a new host.