Cryptosporidiosis is also known as crypto, is a parasitic disease caused by Cryptosporodium, a protozoan parasite of the tribe Apiocomplexa.

 The symptoms manifests itself from two to ten days after infection, with an average of seven days and can last up to two weeks, or in some cases up to a month. Other symptoms include nausea, vomiting and dehydration. The individuals who are asytomatiska (have no symptoms) are still infectious and can therefore transmit the infection to others. Even after symptoms have finally subsided, an individual remains infectious for a few weeks.

When Cryptosporidium spreads beyond the intestine, especially in patients with AIDS, it can reach the lungs, middle ear, pancreas and stomach. The parasite can also infect the biliary tract. Cryptosporidiosis occurs worldwide. It causes 50.8% of the water-borne diseases attributed to parasites

1987 was 13,000 people in Carrolton, Georgia, ill with cryptosporidiosis. This was the first report on the spread of a municipal water systems that met all state and federal standards for drinking water. In March 2007, there were a suspected outbreak in Galway, Ireland. The source for much of the region, Lough Corrib, was suspected of being infected by the parasite. The population of 90,000 people, including areas for both Galway city and surrounding counties were advised to boil water before drinking, cooking and brushing teeth. On 21 March 2007 confirmed that the city and the county's water supply was contaminated by the parasite. Area's water supply finally got an approval on 20 August, five months after the Cryptospridum first detected. Approximately 240 known cases of disease-bearing persons were reported, but according to experts, the real figure would have been right up to In November 2010 reported 4500 cases of cryptosporidiosis in Ostersund. The source of contamination is tap water. Based on the number of cases, estimates the local authorities, the total number affected to between people.

 Cryptosporidium is highly resistant to klordesinfiktion, with enough high concentrations and contact time, disabled Cryptosporidium by chlorine dioxide and ozone treatment. The the required levels of chlorine normally preclude the use of chlorine disinfection as a reliable method to control Cryptosporidium in drinking water. Ultraviolet (UV) light treatment at relatively low doses inactivate Cryptosporidium. Water Research Foundation-paid research discovered the original UV's ability to inactivate Cryptosporidium.

In year 2000 came the first wave of cryptosporidium in Sweden's history. In Ostersund they found the parasite Cryptosporidium in the water and then they had to boil their water for about 3 months before the problem was fixt. The year 2011 it was skellefteå who got to be the other city in swedens history to experienced the cryptosporidium parasite, it took about one-two months for skellefteå to get their water purified.