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Cryptosporidium spp. is a protozoan parasite that causes cryptosporidiosis. Cryptosporidiosis may be asymptomatic, but can also present as a self-limiting disease characterised by watery diarrhoea, often accompanied by dehydration, weight loss, abdominal pain, fever, nausea and vomiting. Gastrointestinal symptoms tend to last between 1-2 weeks, which is longer than commonly seen with most bacterial gastrointestinal infections. Symptoms are generally more severe in immunocompromised individuals, and the infection can be life threatening if the organism invades organs such as the lungs and bile duct.

The two main species associated with human infection are C. parvum and C. hominis, the primary reservoirs of which are livestock (particularly calves and lambs) and humans, respectively. Cryptosporidiosis may be contracted through consumption of food or water that has been contaminated with faecal matter from an infected person or animal. It can also be contracted through recreational bathing or direct contact with contaminated animals or their faeces. Consumption of contaminated water is of particular concern as Cryptospiridium is relatively resistant to chlorine treatment. Public and private water supplies rely on coagulation and filtration mechanisms, or natural purification by geological processes to form a barrier to Cryptospiridium contamination. However, these systems can become overwhelmed during heavy rainfall with the result that numbers sufficient to cause illness can enter the water supply.