The specific aims of this project are to study: 1) the epidemiology of Cryptosporidiosis, a new parasitic enterocolitis. 2) The biological characteristics of the parasite, and 3) its relationship to the host with/without acquired immunodeficiency syndrome. Epidemiological Study-will be conducted with a large stool screening for Cryptosporidium oocysts in the population of New York City, with matched controls in the non-gay population, by examining diarrheal/purged stool samples using direct-saline iodine preparation, direct modified Kinyoun, and Auramine-rhodamine fluorescent preparation and concentration preparation with Sheather's sugar flotation technique. The immediate goal is to determine a) the mode of transmission, b) the carrier rate, c) the prevalence of this enterocolitis, with possible respiratory involvement and d) its environmental distribution. The oocyst(s) isolated from stool examination will be studied as follows: Biological Characteristics of Cryptosporidium-a) the general characteristics of the parasite-optimal growth environment, resistance to physical agents and treatment; b) the mechanism of pathogenicity-virulent factors based on toxin production, invasiveness (in AID's patient), adherence factor, various iso-enzyme production, interaction with bowel microbial flora, (especially virus) and cellular components as well as rate of infectivity; c) in vitro and in vivo drug susceptibility in tissue cultures and animals to search for effective drug therapy for human. Relationship of Parasite to the Human Host-The human host with positive stool examination with/without AIDS will be evaluated for a) anti-Cryptosporidial titer, b) routine admission profile (CBC, sed-rate, urinalysis, immunoglobins, complement, VDRL); c) special immunologic profile to determine acquired immunodeficiency syndrome (T lymphocyte response to mitogens, natural killer cell activity, B cell allo-antigens, T lymphocyte subsets and circulating immune complexes, etc.; d) the presence of underlying diseases or other opportunistic infections (Pneumocystosis, etc.). Separate protocol on drug trials on human subjects will be submitted at a later date when animal and tissue culture test results warrant it. In conclusion, the long term goals are 1) to develop simple laboratory diagnostic tests, stool examination and serologic test for diagnosis, 2) effective therapy for human, and 3) prevention and specific enteric precaution directive and possible vaccine prophylaxis in high risk individuals.