In the Spring of 1993, almost 400,000 people living in Milwaukee, Wisconsin were exposed to a serious biological contaminant in the drinking water by their Milwaukee municipal water authority. 
An alert physician, noticing large numbers of people with similar but unusual symptoms, raised the alert and advised the authorities.  The cause was determined a week after the initial exposure. The population had been drinking water with the biological parasite Cryptosporidium.  This contamination was not visible, and the water seemed to be normal.

Many people were taken to local hospitals with severe diarrhea and other symptoms, and at hospital were given more of the water that caused the problems in the first place as some hospitals were using city water too.  More than 100 died.

In a follow up investigation the Milwaukee Journal reported:
“Government regulators knew Cryptosporidium lurked dangerously in drinking water but took no action.”
“Cryptosporidium is still causing long term medical problems and deaths” (the MJ report was  several months after the initial exposure)
“Micro-organisms in water may cause up to a third of all US diarrheal illness”
“Milwaukee’s epidemic has cost more than $54 million”

Milwaukee Journal Special Report September 19-26 1993.

A number of lessons can be learned from the Milwaukee incident.

• Dangerously polluted water can impact on people on a mass scale.
• The water was delivered through the local municipal water system.
• The water utility did not detect or stop the contamination.
• It was a week after the initial exposure before the problem was identified.
• People who were admitted to hospital due to the problem were given the same water that put them there in many cases.