Days 1 - 21, Arm I will receive oral NTZ and Arm II will receive NTZ placebo po bid (blinded). With the approval of the protocol chair, patients may switch to open-label NTZ after two weeks of blinded therapy if there is a clinical worsening of diarrhea due to cryptosporidiosis accompanied by either weight loss >= 5% or the requirement for intravenous fluids to maintain body weight and/or intravascular volume despite the use of appropriate antidiarrheal agents.