Isospora, genus of parasitic protozoans of the sporozoan subclass Coccidia. Isospora causes the disease known as coccidiosis (q.v.) in humans, dogs, and cats. The species that attack humans, I. hominis and I. belli, inhabit the digestive tract and are endemic in many areas of southern Europe, Asia, Africa, Latin America, and Oceania. Symptoms of human infection include weight loss, digestive disturbances, and fever. Dogs and cats are infected by the species I. bigemina, I. rivolta, and I. felis. The genus is recognized by spore cases containing two spores, each of which contains four infective parasites (sporozoites).