Cryptosporidium parvum

Cryptosporidium is a leading cause of diarrheal disease in young children and untreated AIDS patients in resource-limited countries worldwide. Transmission occurs via the fecal–oral route, and sources of Cryptosporidium infection include contaminated water or food, or contact with infected people or animals. Upon ingestion of the infective parasite oocysts, motile sporozoites emerge and invade epithelial cells of the small intestine where they develop in an intracellular but extracytoplasmic niche. Cryptosporidium completes its complex life cycle in a single host, with both asexual and sexual stages present in the intestine. Replication of the parasite, and the resulting immune response contribute to the development of severe, watery diarrhea in infected individuals. Currently, there is no vaccine, and only one drug (nitazoxanide), which has limited efficacy in those most susceptible.