Coccidial parasites of the genus Isospora cause intestinal disease in several mammalian host species. These protozoal parasites have asexual and sexual stages within intestinal cells of their hosts and produce an environmentally resistant cyst stage, the oocyst. Infections are acquired by the ingestion of infective (sporulated) oocysts in contaminated food or water. Some species of mammalian Isospora have evolved the ability to use paratenic (transport) hosts. In these cases, infections can be acquired by ingestion of an infected paratenic host. Human intestinal isosporiasis is caused by Isospora belli. Symptoms of I. belli infection in immunocompetent patients include diarrhea, steatorrhea, headache, fever, malaise, abdominal pain, vomiting, dehydration, and weight loss, blood is not usually present in the feces. The disease is often chronic, with parasites present in the feces or biopsy specimens for several months to years. Recurrences are common, Symptoms are more severe in AIDS patients, with the diarrhea being more watery. Extraintestinal stages of I. belli have been observed in AIDS patients but not immunocompetent patients. Treatment of I. belli infection with trimethoprim-sulfamethoxazole usually results in a rapid clinical response. Maintenance treatment with trimethoprim-sulfamethoxazole is needed because relapses often occur once treatment is stopped.