Protozoa of the phylum Microspora are obligate intracellular pathogens that are being detected with increasing frequency in humans, especially in patients with acquired immunodeficiency syndrome. Organisms from four genera have been reported to date, and serological data suggest the occurrence of latent infections. Sources of human infections are not known, but microsporidia are widespread in lower vertebrates and invertebrates. There is no known treatment. Study of the disease in mammals suggests that infection often will be clinically silent, that intact T-cell-mediated host defenses are required for resistance, and that serious clinical disease may occur under circumstances in which extensive parasite replication can occur.