This chapter is designed to discuss Cryptosporidium parvum in context of its infectivity, pathogenesis, and immunity. Cryptosporidium has been found in human and animal populations worldwide, and it is responsible for causing human melancholy in both developing as well as developed nations. This parasitic protozoan is particularly dangerous in people who have chronic disease, malnutrition, or other debilitating conditions that lead to compromised immune systems. The disease may be chronic and even life-threatening for undernourished infants as well as AIDS patients. Further, the graveness of the infection is compounded by the lack of curative therapy. In comparison, immune-competent individuals may often produce a profuse, watery diarrhea accompanied by abdominal pain and other enteric symptoms; however, the disease in this population is self-limited. Outbreaks of cryptosporidiosis in developed countries have been associated with waterborne transmission via recreational and drinking water supplies. However, because the pathogen can also be transmitted directly from an infected person to susceptible individuals, it is possible that the majority of infections contracted in non-outbreak situations may be the result of this route. Thus, labors have been expended in understanding the basic biology and epidemiology of the organism. The purpose of this chapter is to review the findings that have come from the human volunteer studies and to summarize these data in the context of the current comprehension of Cryptosporidium infectivity, pathogenesis, and immunity.