The Drinking Water
Water is not the most likely suspect.
In developed countries, very few E. coli outbreaks are spread through water; the vast majority are carried on foodstuffs. However, as a precaution, since a small percentage of cases are carried through water, on May 19, the day the investigation begins, the public health unit contacts the manager of the municipal water system to make sure that no unusual results have turned up during routine water-testing. The unit is assured that the water has been "OK", which seems to confirm that investigators should be searching for a food item contaminated with E. coli.
Furthermore, the people falling sick at Walkerton have not been drinking untreated water; their water comes from a municipal system, which is supposed to have been designed, and to be treated and monitored, to meet provincial standards.
Therefore, the health unit focuses its first investigations on
