An intestinal infection caused by the protozoan parasite Cystoisospora belli (formerly called Isospora belli). Isosporiasis can be spread by ingesting food or water contaminated with the parasite. Symptoms of isosporiasis include watery diarrhea, abdominal pain, anorexia, and low-grade fever. In people with HIV, chronic isosporiasis (lasting over 1 month) is an AIDS-defining condition.