Although some strains of the bacteria are harmless, others can cause diarrhea, urinary tract infections, respiratory illness and pneumonia.

One harmful strain is the Shiga toxin-producing E. coli, also known as STEC, which live in the guts of animals such as cattle, goats, sheep, deer and elk. Humans can become infected by eating contaminated food, drinking raw milk or contaminated water, coming in contact with cattle or with the feces of infected people.

STEC are resistant to a number of classes of antibiotics. In fact, antibiotic treatment is generally discouraged because it may increase the risk of developing hemolytic uremic syndrome, a disorder that can destroy red blood cells, causing damage to the kidneys.

An estimated 265,000 STEC infections occur yearly in the U.S., reports the CDC.