An outbreak of E. coli O157:H7 and Campylobacter infections was identified by the Bruce-Grey-Owen Sound Health Unit, Ontario, Canada, in May 2000. While most of the ill lived in Walkerton, a number of the ill lived in outlying communities. The water supply was suspected after an exhaustive investigation failed to identify any common food exposure or community event that would explain all of the illnesses. Despite reassurances that the Walkerton Public Utilities water was safe and secure, the health unit issued a boil water order on May 21. Subsequent testing of the water confirmed the presence of E. coli and coliform bacteria. Persons who consumed the water were 11.7 times more likely to develop gastroenteritis than those who had not drunk Walkerton water; people who consumed more water were more likely to become ill. Within five days of the boil water order, the case reports began to decline. E. coli O157:H7 was eventually confirmed in the water. One of the wells in use at the time of the outbreak was known to be subject to surface water contamination. Heavy rains and the presence of E. coli O157:H7 and Campylobacter on farms adjacent this well probably led to the contamination of the water. A follow up study of those who had become ill due to this outbreak showed that 36% subsequently developed irritable bowel syndrome.