In late 2005, health officials in Clark County, Washington noted a higher than expected number of Escherichia coli cases among residents and sought to identify a possible common source for infection. In order to identify risk factors, health officials conducted a retrospective cohort study and an environmental site investigation using pulsed-field gel electrophoresis (PFGE) to identify and prevent future cases from occurring. Several lines of evidence supported raw milk as the cause of infections: 1) all ill persons drank raw milk from the same cow share; 2) illness was associated with an increasing amount of milk consumed; 3) E. coli O157:H7 was isolated from raw milk samples and environmental samples collected from the floor of the milking parlour; and 4) PFGE patterns were indistinguishable between case-patients, raw milk samples, and environmental samples. Together, these findings made clear the health risks associated with the consumption of raw milk. The high amount of media interest in this investigation empowered public health officials to work with state legislators to pass State Senate Bill 6377 in Washington State which clarified that state licensing requirements apply to all milk production facilities including cow-share programs.