The coccidian parasite, Isospora belli , infects the epithelial cells of the small intestine, and is the least common of the three intestinal coccidia that infect humans. Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication . Trophozoites develop into schizonts which contain multiple merozoites. After a minimum of one week, the sexual stage begins with the development of male and female gametocytes .
Fertilization results in the development of oocysts that are excreted in the stool .
Isospora belli infects both humans and animals. Worldwide, especially in tropical and subtropical areas. Infection occurs in immunodepressed individuals, and outbreaks have been reported in institutionalized groups in the United States.