Causal Agent:
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.

Life Cycle:

At time of excretion, the immature oocyst contains usually one sporoblast (more rarely two) .  In further maturation after excretion, the sporoblast divides in two (the oocyst now contains two sporoblasts); the sporoblasts secrete a cyst wall, thus becoming sporocysts; and the sporocysts divide twice to produce four sporozoites each .  Infection occurs by ingestion of sporocysts-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogony .  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.

Geographic Distribution:
Worldwide, especially in tropical and subtropical areas.  Infection occurs in immunodepressed individuals, and outbreaks have been reported in institutionalized groups in the United States.