GI tract infection caused by the protozoan parasite Cryptosporidium.Encyclopedia Entry for Cryptosporidiosis :Cryptosporidiosis: Cryptosporidiosis has become one of the most common causes of waterborne disease in the United States in recent years; it is also found throughout the rest of the world. It is caused by a parasite that spreads when a water source is contaminated, usually with the feces of infected animals or humans. Symptoms include diarrhea, stomach cramps, an upset stomach, and slight fever. Some people do not exhibit any symptoms.Encyclopedia Entry for Cryptosporidiosis :Cryptosporidiosis. Source of disease: Cryptosporidium species