Cystoisospora belli (formerly Isospora belli) infection is acquired by oocyst ingestion and is most common in tropical and subtropical countries. Acute infection can begin suddenly with fever, abdominal pain, and watery, nonbloody diarrhea and can last for weeks or months. Eosinophilia may occur. Compromised (e.g., HIV-infected) pts may have chronic disease that resembles cryptosporidiosis. Detection of large oocysts (~25 µm) in stool by modified acid-fast staining confirms the diagnosis.