Cryptosporidium is a small protozoan parasite, measuring about 3-5 µm. It lives on (or just under) the surface of the cells lining the small intestine, reproduces asexually, and oocysts are passed in the faeces. Transmission of the infection occurs via the oocysts. Many human infections have been traced to the contamination of drinking water with oocysts from agricultural "run-off" (i.e., drainage from pastures), so it is considered a zoonosis. The most common symptom of cryptosporidiosis is watery diarrhoea; other symptoms include stomach cramps or pain, fever, nausea and vomiting. In persons with healthy immune systems, symptoms usually last about 1 to 2 weeks. Cryptosporidiosis is now recognized as an important opportunisitic infection, especially in immunocompromised hosts