Project Summary Cryptosporidiosis is a leading contributor to child morbidity and mortality worldwide, responsible for over 200,000 deaths in children under two, and morbidity in over 7 million children. However, we have limited treatment and no vaccine for use in young children. Our prior work in Bangladesh has demonstrated that almost 80% of children in an urban slum cohort were infected with Cryptosporidium spp. by age two and that infection, even without diarrhea, was significantly associated with long-term growth faltering. Nitazoxanide is the only FDA-approved drug for treatment of Cryptosporidium diarrheal disease. However, the drug has limited widespread due to lack of large-scale, unbiased rigorous trials demonstrating its efficacy in children under one and establishing its safety in this population. Furthermore, little is known about how the drug could treat non-diarrheal infection and alleviate Cryptosporidium’s effect on child growth. Here we propose a double-blind, randomized, placebo-controlled trial aimed at treating children ages 6-12 months with Cryptosporidium-diarrheal and non-diarrheal infection. The study will evaluate the efficacy of treatment for both conditions and will also evaluate whether treatment can improve growth in children for 6 months post-treatment. This proposal leverages a longstanding collaboration between Johns Hopkins University and the International Centre for Diarrhoeal Disease, Research, Bangladesh. This proposal is innovative, as it will be the first randomized controlled trial aimed at treatment of non-diarrheal Cryptosporidium infection and will evaluate impact on growth. The findings from this study will inform treatment efforts aid to protect young children from this devastating infectious disease.