McDonald's takes Quarter Pounder off the menu at 1 in 5 restaurants due to E. coli outbreak

McDonald's has stopped selling Quarter Pounder hamburgers at about one-fifth of its U.S. restaurants as federal health officials investigate an outbreak of E. coli that has sickened nearly 50 people in 10 states, killing one.

The fast-food chain sold about 1 million Quarter Pounders in the timeframe that the illnesses occurred, a McDonald's spokesperson said Wednesday. The company said it believes more people would have been sickened if the contamination stemmed from the patties themselves, adding that onions used on the burgers may be the culprit.

Burgers sold by McDonald's are cooked at 175 degrees, above the 160 degree level needed to kill the E. coli bacteria, according to the spokesperson.