Eimeria, genus of parasitic protozoans of the spore-producing phylum Apicomplexa (previously Sporozoa). Eimeria, which causes coccidiosis in livestock and wild animals, infects mainly the cells of the digestive tract, although it also attacks cells of the liver and the bile duct. Symptoms of infection are diarrhea, weight loss, and general weakness. Eimeria is characterized by spore cases that contain four spores, each with two infective sporozoites. Among the common pathogenic species are E. necatrix and E. tenella (in poultry); E. stiedae (in rabbits); and E. bovis, E. ellipsoidalis, and E. zuernii (in cattle).