Escherichia coli bacteria exist naturally in the intestines of humans and animals. They help break down nutrients. However, there is a form which exists in the intestines of cows and other ruminants named enterohaemorrhagic escherichia coli, which produces toxins. If this infects food, it causes diarrhea, nausea, and stomach pains.
The source of the infection may be raw or insufficiently cooked meat, raw or insufficiently heated milk or drinking water contaminated with feces. Avoidance of these sources and compliance with hygienic principles (transmission from person to person is possible) can prevent poisoning.