Patients receive oral nitazoxanide daily for 14 days, after which those with complete clinical and parasitologic response discontinue treatment. Non-responders and partial responders may continue therapy for an additional 14 days at the discretion of the physician. Non-responders who show signs of improvement or who have partial response after 28 days and those who relapse following complete response may continue therapy for an additional month, up to 60 days total. [AS PER AMENDMENT 10/30/96: Patients receive a daily treatment for 4 weeks, with subsequent dose escalation in the absence of drug-related toxicity. Patients who exhibit complete response after 2 months may continue at a maintenance dose. Patients enrolled after October 15, 1996 are randomized to 1 of 2 doses, with subsequent escalations made in the absence of toxicity. Complete responders may continue therapy at a maintenance dose and duration determined by the investigator. Non-responders after 6 months of therapy have treatment discontinued.] [AS PER AMENDMENT 8/5/97: All patients are evaluated at Weeks 1, 2, 4, and monthly thereafter.] [AS PER AMENDMENT 8/17/99: New patients start therapy on a different dosage of nitazoxanide. Those who do not respond after 4 weeks of therapy will escalate to a higher dosage. Patients who show a complete response at 2 consecutive visits (2 weeks apart) discontinue nitazoxanide therapy and go to follow-up.] [AS PER AMENDMENT 2/3/00: The Week 1 clinical evaluation is deleted from the study procedures.]