In May 2000, bacterial contamination of municipal water in Walkerton, Ontario, led to Canada’s worst public health disaster involving municipal water. At least seven people died, and 2,300 fell ill from gastroenteritis caused by the contamination.

The outbreak, involving E. coli and Campylobacter jejuni bacteria, occurred after heavy rainfall washed bacteria from cattle manure used as fertilizer into the shallow aquifer of a nearby well. Inadequate water treatment allowed the contamination to enter the drinking water supply, with the first case reported on May 17.