Escherichia coli or E. coli are a large and diverse group of bacteria. Although most strains of E. coli are harmless or even beneficial, others can make you sick and can even be deadly. Some kinds of E. coli can cause bloody diarrhea, while others cause urinary tract infections, respiratory illness and pneumonia, and other illnesses. Still other kinds of E. coli are used as markers for water contamination—so you might hear about E. coli being found in drinking water, which are not themselves harmful, but indicate the water is contaminated.
