Isosporiasis is an uncommon diarrheal illness caused by the protozoan Isospora belli. Humans are the only known
hosts for I belli, and there is no known animal reservoir. Isospora has worldwide distribution, although it is more
common in tropical and subtropical climates. In 1860, Virchow first described Isospora. In 1915, the first case of human
infection with I belli was described. Isospora is related closely to Cryptosporidium, Cyclospora, and Toxoplasma.

The oocysts of Isospora are resistant and remain viable in the environment for months. Ingestion of mature oocysts of
Isospora leads to invasion of the epithelial cells of the distal duodenum and proximal jejunum, with resulting cell
damage. Symptoms of isosporiasis suggest a toxin-mediated mechanism, but no toxin has been identified. In humans,
extraintestinal forms of the disease are rare but have been reported in patients with AIDS.

United States
The exact incidence of isosporiasis in humans is not known, but Isospora has been reported as the cause of
outbreaks of diarrheal illness in day care centers and mental institutions. I belli has been implicated in traveler's
diarrhea in endemic areas.