The U.S. Centers for Disease Control and Prevention issued a food safety alert Tuesday regarding an outbreak of E. coli. coli that they believe is linked to McDonald’s Quarter Pounders.

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

Most of the people who got sick reported eating McDonald’s Quarter Pounder sandwiches, according to the CDC.

The agency says the investigation is “advancing rapidly” but has not identified a specific ingredient. However, investigators are focusing on two ingredients: sliced ​​onions and beef patties. According to the agency, beef patties are used only for Quarter Pounders, and onions are primarily used for Quarter Pounders and not other items.

McDonald’s said in a statement that the initial investigation’s findings linked the ingredients to “a single supplier that serves three distribution centers.”

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

According to the CDC, McDonald’s has removed the ingredients from stores in Colorado, Iowa, Kansas, Missouri, Montana, Nebraska, Oregon, Utah, Wisconsin and Wyoming. In other states, Quarter Pounder burgers may not be available.

However, the CDC also notes that the outbreak could expand beyond these states.

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

Anyone who becomes ill after eating a quarter-pound of McDonald’s should see a doctor and tell the provider what they ate, according to the CDC.

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

McDonald’s stock price fell precipitously after hours as news of the outbreak broke.

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