The number of cases is likely much higher than reported because most people with E. coli infections recover on their own and are never tested for the bacteria.

E. coli symptoms — which include vomiting, diarrhea and a fever of at least 102 degrees — generally begin three to four days after eating contaminated food.

The Food and Drug Administration and the CDC have identified the onion slices served on the hamburgers as a likely source of contamination. although health officials have not ruled out the beef patties used in the Quarter Pounders.

On Thursday, McDonald's confirmed that Taylor Farms, a California-based food producer, supplied the onions for the contaminated Quarter Pounders.

Taylor Farms recalled four raw onion products Wednesday due to “potential E. coli contamination.” Several restaurant chains in Colorado — including Taco Bell, Burger King, KFC, Pizza Hut and Illegal Pete's — said they removed onions from their menus out of an abundance of caution following the recall. The FDA said restaurant chains that received the recalled onions were notified directly. There is no evidence of illness associated with these restaurants.

McDonald's has removed the offending onion slices and Quarter Pounder patties from its restaurants in at least 12 states: Colorado, Kansas, Utah, Wyoming, Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma.

It remains unclear whether Taylor Farms transported the sliced ​​onions directly to McDonald's locations or whether they arrived through a distributor. Taylor Farms did not respond to multiple requests for comment.

McDonald's is being sued by two people who said they became ill after eating there.

The plaintiffs in the two lawsuits are both represented by Ron Simon, managing partner of Ron Simon & Associates, a food safety law firm. Simon told NBC News on Thursday that he represents a total of 15 people who say they became ill in the outbreak.