I. belli is common in tropical and subtropical environments. Sporadic outbreaks have occurred in mental institutions and in day-care centers in the United States. The infection is common in immunosuppressed patients, particularly those with AIDS living in tropical areas. Transmission probably occurs by the fecal-oral route. Chronic diarrhea is the major clinical manifestation. The pathophysiology of I. belli-induced diarrhea is unknown. Laboratory diagnosis is readily performed with the examination of unstained and stained stool specimens particularly by the modified Ziehl-Nielsen method. I. belli infection responds promptly to trimethoprim-sulfamethoxazole therapy and recurrent disease can be prevented with either trimethoprim-sulfamethoxazole or pyrimethamine-sulfadoxine.