Cystoisosporiasis (formerly known as isosporiasis) is an intestinal disease of humans caused by the coccidian parasite Cystoisospora belli (formerly known as Isospora belli). Cystoisosporiasis is most common in tropical and subtropical areas of the world. The parasite can be spread by ingesting contaminated food or water. The most common symptom is watery diarrhea. The infection is treatable and preventable.