The gut Coccidia are members of a large, varied, and exclusively intracellular group of protozoan parasites, four species of which (Isospora, Cryptosporidium, Cyclospora, and Sarcocystis) are human pathogens. The first three, but particularly Cryptosporidium parvum, have moved from medical curiosities to major problems with the coming of the acquired immunodeficiency syndrome (AIDS) epidemic, but are now known to also cause disease in the immunocompetent patient. They are easy to acquire and difficult to remove from the environment and, in the case of cryptosporidiosis, impossible to treat properly. Further research into many aspects of the biology of these organisms is urgently needed.