Since 1976, when Cryptosporidium was first recognized as a human pathogen, understanding of the epidemiology of this protozoan parasite has increased substantially. This review discusses 14 recently published studies of t,he occurrence of cryptosporidiosis in developed and developing countries and compares their findings with those of previous epidemiologic reports. These stu.dies showy that cryptosporidios,is is an important publ,ic health problem worldvcide. Although cryptosporidiosis was first recognized as a problern in immunosuppressed patients, persons with normal immunologic function are also affecte& This appears to be especially true for children in developing countries; of 1035 children with diarrhea reported in several surveys, 79 (7.6%) had Cryptosporidium. Findings from studies that included both asymptomatic as well as symptomatic persons have demonstrated that Cryptosporidiun; infections are rare in persons without symptoms, suggesting that Cryptosporidium should not be thought of as an opportunistic parasite. Early studies documented the potential for animal-to-human transmission of Cryptosporidium, but it is now clear that many, perhaps most, Cryptosporidium infections in humans are not acquired directly from infected animals. Although alternate modes of transmission are just beginning to be explored, evidence indicates that person-to-person spread is important. Since 1976, when Cryptosporidium was first recognized as a h u m a n pathogen (27), understanding of this protoz.oan parasite has increased * Hospital Β© Laboratory o Survey = hospitalized patients. = persons submitting stool specimens to laboratory for routine diagnosis. = prospective survey of diarrhea in predefined population.