Cryptosporidiosis is an infectious disease caused by the coccidian parasite Cryptosporidium spp. that primarily infects the gastrointestinal tract of animals. Prior to late 1982, only 11 cases of human infection had been reported, with the first human case reported in 1976. During the 1980s, the number of human cases began to rise dramatically. Most of these patients were immunodeficient, many of whom had the acquired immune deficiency syndrome (AIDS). Immunocompetent individuals can also acquire the infection with mild-to-severe diarrhea lasting from several days to weeks. Immunocompromised patients develop severe, irreversible diarrhea, often thought to be a significant contributing factor leading to death. Although many therapeutic compounds have been tried, none have proven to be very successful. The incidence of this infection is worldwide, with many published reports of infection in both immunodeficient and immunocompetent individuals. The diagnostic approach has been expanded to include stool examinations using modified acid fast procedures and fluorescent monoclonal antibody reagents. Although histological methods are still routinely used for biopsy specimens, the examination of stool has proven to be a very effective, noninvasive procedure. In addition to the gastrointestinal tract, other areas of the body that have been found to be infected with this organism include the respiratory tract and the biliary tree and gall bladder epithelium. Screening for this organism may become very important in known risk groups (animal handlers; children, staff members, and contacts of those who attend day care centers; travelers; and those who are immunodeficient), particularly if an effective therapy is found. Some laboratories screen every specimen submitted for an ova and parasite examination, while others have limited their testing to specific requests, risk groups, and those patients who are symptomatic.