These are parasites that must be swallowed to cause disease. The organisms are spread by the fecal-oral route. The illness caused by Cryptosporidium is characterized by watery diarrhea and abdominal cramps. The illness caused by Giardia is characterized by nausea and fatigue, followed by watery diarrhea that can last more than 10 days. These organisms are measured in source waters for drinking water systems that use surface water.