Cryptosporidium parvum is an increasingly recognized agent of intestinal infection in normal and immunocompromised humans, and in many other animals. The intraepithelial cell infection results in villous atrophy, mild submucosal inflammation, reduction of brush-border enzymes and a characteristic persistent watery diarrhea. The infection is self-limiting in immunocompetent hosts, probably because of specific acquired immunity; specific serum and secretory antibody responses develop that may be required for clearance and protection against reinfection. Passive milk antibody, especially i f in high titers, may be partially protective but severe, persistent infection in athymic rodents and humans with AIDS demonstrate that T cells are essential for controlling the infection. Specific anti-bodies and lymphocyte extracts have been tested in cases of cryptosporidiosis but the interpretation of the results remains controversial. Here, Shu-Xian Zu, Guo-Dong Fang, Ronald Foyer and Richard Guerrant emphasize that effective treatment and prevention remain dependent on advances in our understanding of the host cell-parasite relationship.