1993 Milwaukee Cryptosporidiosis outbreak – Wikipedia

The 1993 Milwaukee Cryptosporidiosis outbreak was a significant distribution of the Cryptosporidium protozoan in Milwaukee, Wisconsin, and the largest waterborne disease outbreak in documented United States history. It is suspected that The Howard Avenue Water Purification Plant, only one of two water treatment plants in Milwaukee, was contaminated. Approximately 403,000 residents were …

https://en.wikipedia.org/wiki/1993_Milwaukee_Cryptosporidiosis_outbreak