CNN
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The U.S. Centers for Disease Control and Prevention issued a food safety alert Tuesday regarding an E. coli outbreak said to be linked to McDonald's Quarter Pounders.

At least 49 illnesses have been reported in 10 states, including one death in an elderly person. Ten people were hospitalized, including a child suffering from hemolytic uremic syndrome, a serious complication that can develop from an E. coli infection. Most diseases occur in Colorado and Nebraska.

According to the CDC, most people who got sick reported eating McDonald's Quarter Pounder sandwiches.

The agency says the investigation is “moving quickly” and that information reviewed by the U.S. Food and Drug Administration shows chopped onions are a likely source of contamination.

According to the CDC, McDonald's has stopped using onions and quarter-pound beef patties in several states while the investigation continues. According to the agency, the beef patties are used only for the Quarter Pounders, and the chopped onions are used primarily for the Quarter Pounder and not for other products.

McDonald's said in a statement that initial investigative results linked the onions to “a single supplier serving three distribution centers.”

“We are working closely with our suppliers to replenish Quarter Pounder supplies in the coming weeks (timing varies depending on local market),” it said. “In the meantime, all other menu items, including other beef products (including Cheeseburger, Hamburger, Big Mac, McDouble and Double Cheeseburger), are unaffected and available.”

According to the FDA, McDonald's has removed the onion slices and quarter-pound patties from stores in Colorado, Kansas, Utah, Wyoming and parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma. Quarter Pounder burgers may not be available in other states.

However, the CDC also notes that the outbreak may extend beyond these states.

People with E. coli infections may experience symptoms such as severe stomach cramps, diarrhea, fever, and vomiting. Symptoms of infection usually begin three to four days after the bacteria is swallowed. Although most sick people recover within a week without treatment, others may develop serious kidney problems and require hospitalization.

Anyone who becomes ill after eating a McDonald's Quarter Pounder should seek medical attention and tell the provider what they ate, the CDC says.

McDonald's said in its statement that “safely serving customers in every single restaurant, every day, is our top priority” and that it will continue to cooperate with the CDC in the investigation.

McDonald's stock price fell sharply in after-hours trading as news of the outbreak broke.