Escherichia coli is a common bacterium that is normally present in the human intestine. There are many different types, or strains, of E coli that infect many types of animals and most strains are harmless. A few strains can cause illness. These strains are often found in animals such as cattle, goats, and sheep and do not cause illness in these animals. The most common cause of E coli infection in humans is ingesting contaminated food or water or having contact with infected animals.