The Commissioner of Health was astonished when businesses in Milwaukee were closing down due to mass people reporting symptoms of diarrhea and gastrointestinal illnesses. A water-borne parasite was identified as the culprit after analysis of stools and water supplies. The mayor quickly issued a boil water advisory that seemed to help prevent future outbreaks. The April 1993 outbreak is the largest water-borne disease outbreak ever documented in the US. More than 400,000 of the city's 1.6 million people became ill and several people died.