According to the Centers for Disease Control and Prevention website, Escherichia coli (abbreviated as E. coli) are bacteria found in the environment, foods, and intestines of people and animals. E. coli are a large and diverse group of bacteria. Although most strains of E. coli are harmless, others can make you sick. According to the Centers for Disease Control and Prevention, E. coli bacteria normally live in the intestines of people and animals. Most E. coli are harmless and actually are an important part of a healthy human intestinal tract. However, some E. coli are pathogenic, meaning they can cause illness, either diarrhea or illness outside of the intestinal tract. The types of E. coli that can cause diarrhea can be transmitted through contaminated water or food, or through contact with animals or persons. An article published by the EPA discusses how E. coli was determined as a water quality parameter and how the standards were set.
You can find more resources about E. coli by following the following links:
Centers for Disease Control and Prevention
Indiana Department of Environmental Management