Cryptosporidium parvum is an intracellular parasite that causes gastrointestinal disease. In humans, it causes cryptosporidiosis, which is manifested in nausea, vomiting, diarrhea, occasional shortness of breath, coughing, and elevated temperature. Infection occurs after ingestion of oocysts. The infectious dose for healthy people is about 30 oocysts. For people with reduced immunity, only one cyst may be sufficient for successful infection and the course of the disease is much more severe, sometimes resulting in death. Oocysts are relatively resistant, some of them can survive water purification. These protozoa were the cause of many epidemics, mainly in the USA and in countries with poor sanitary and epidemiological conditions. Occasionally recurring epidemics in the United States have caused interest in the problem of scientists and the government of this country. This resulted in the introduction of mandatory detection of this protozoan cyst in drinking water.