An estimated 2,300 people became seriously ill and seven died from exposure to microbially contaminated drinking water in the town of Walkerton, Ontario, Canada in May 2000. The severity of this drinking water disaster resulted in the Government of Ontario calling a public inquiry by Mr. Justice Dennis O'Connor to address the cause of the outbreak, the role (if any) of government policies in contributing to this outbreak and, ultimately, the implications of this experience on the safety of drinking water across the Province of Ontario. The circumstances surrounding the Walkerton tragedy are an important reference source for those concerned with providing safe drinking water. Although some circumstances are obviously specific to this epidemic, others are uncomfortably reminiscent of waterborne outbreaks that have occurred elsewhere. These recurring themes suggested the need for attention to broad issues of drinking water security and they present the challenge for how drinking water safety can be managed to prevent such tragedies in the future.

Millions of people die every year around the world from diarrheal diseases much of which is caused by contaminated drinking water. By contrast, drinking water safety is largely taken for granted by many citizens of affluent nations. The ability to drink water that is delivered into households without fear of becoming ill may be one of the key defining characteristics of developed nations in relation to the majority of the world. Yet there is well-documented evidence that disease outbreaks remain a risk that could be better managed and prevented even in affluent nations. A detailed retrospective analysis of more than 70 case studies of disease outbreaks in 15 affluent nations over the past 30 years provides the basis for much of our discussion [Hrudey, S.E. and Hrudey, E.J. Safe Drinking Water--Lessons from Recent Outbreaks in Affluent Nations. London, UK: IWA Publishing; 2004.]. The insights provided can assist in developing a better understanding within the water industry of the causes of drinking water disease outbreaks, so that more effective preventive measures can be adopted by water systems that are vulnerable. This preventive feature lies at the core of risk management for the provision of safe drinking water.

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Residents of affluent nations are remarkably lucky to have high-quality, safe drinking water supplies that most residents of modern cities enjoy, particularly when considered in contrast to the toll of death and misery that unsafe drinking water causes for most of the world's population. Some may presume that drinking-water disease outbreaks are a thing of the past, but complacency can easily arise. A review of drinking water outbreaks in developed countries over the past 3 decades reveals some of the reasons why drinking water outbreaks keep occurring when society clearly has the means to prevent them. Prevention of future outbreaks does not demand perfection, only a commitment to learn from past mistakes and to act on what has been learned. Water Environ. Res., 79, 233 (2007).

An estimated 2300 people became seriously ill and 7 died from exposure to microbially contaminated drinking water in the town of Walkerton, Ontario, in May of 2000. The severity of this drinking water disaster resulted in the Government of Ontario calling a public inquiry by Mr. Justice Dennis O'Connor to address the cause of the outbreak, the role, if any, of government policies in contributing to this outbreak, and ultimately, the implications of this experience on the safety of drinking water across the province of Ontario. This paper summarizes relevant evidence presented at the inquiry by the expert panel together with Justice O'Connor's findings addressing the circumstances of the outbreak. These findings are reviewed in relation to the published causes of previous waterborne disease outbreaks that have been reported elsewhere in developed countries. The circumstances surrounding the Walkerton tragedy are an important source of knowledge for those concerned with providing safe drinking water to the public. Although some circumstances are obviously specific to this epidemic, others echo common themes in waterborne outbreaks that have occurred before. These common themes suggest the need for attention to broad issues of drinking water safety in addition to the individual specific details that often command attention. Key words: waterborne disease, Escherichia coli O157:H7, Campylobacter jejuni, Cryptosporidium, Giardia, multiple barriers, health risk.