Cryptosporidiosis is an illness caused by the protozoan Cryptosporidium, a single-celled parasite. The most common symptom is diarrhea that is usually watery and profuse, and often accompanied by abdominal cramping. Nausea, vomiting, fever, headache, and loss of appetite may also occur. Rarely, the parasite can cause an inflammation of the gall bladder or infect the lining of the respiratory tract causing pneumonia. Some persons infected with Cryptosporidium may not become ill.