Cystoisosporiasis, also known as isosporiasis, is an uncommon cause of protozoan diarrhea among immunocompetent patients. It is primarily found in immunocompromised patients or individuals visiting endemic areas. Humans are the only known reservoir for the intestinal parasite Cystoisospora belli. This infection is most common in tropical climates with poor sanitation. Contaminated food and water are often sources of infection. Patients of any age are susceptible to infection, and incidence in males and females is similar.

Cystoisospora belli has been associated with outbreaks of diarrheal disease in daycares and institutional settings. In patients with immune compromise, such as in patients with AIDS, leukemia, lymphoma, or solid organ transplantation, it can cause chronic diarrhea, though widespread use of trimethoprim-sulfamethiazole for Pneumocystis jiroveci prophylaxis has reduced the prevalence of Cystoisospora.
    
Symptoms can range from self-limiting diarrhea to life-threatening dehydration. Maintaining hydration is important. Immunocompetent patients respond quickly to therapy, with a less rapid response in immunocompromised patients. Prophylaxis may be required in immunocompromised patients to prevent relapse.