At least 49 people fell ill after the E. coli outbreak linked to McDonald’s quarter pounders, health officials say.

An E. coli outbreak linked to McDonald’s quarter pounder burgers has sickened dozens of people in the United States, including one fatally, health officials said.

At least 49 people have fallen ill in 10 states, with most cases concentrated in Colorado and Nebraska, the U.S. Centers for Disease Control and Prevention said Tuesday.

Most people who became ill reported eating the quarter pounder hamburger at McDonald’s before becoming ill, the CDC said.

Health authorities have yet to determine which specific ingredient may have caused the outbreak.

“McDonald’s is working with research partners to determine which food ingredient in Quarter Pounders is making people sick,” the CDC said in a statement.

“McDonald’s has stopped using fresh-cut onions and quarter-pound beef patties in several states while investigations continue to identify the ingredient causing the illness.”

McDonald’s shares fell about 9 percent in after-hours trading.

McDonald’s U.S. President Joe Erlinger said most states and most menu items were not affected.

“Food safety is so important to me and everyone at McDonald’s,” Erlinger said in a video statement.

“We have taken steps to proactively eliminate the chopped onions used in Quarter Pounders in select states. We have also made the decision to temporarily remove the Quarter Pounder from restaurants in select states.

“We will continue to inform you about the actions we are taking,” Erlinger added.

“At McDonald’s you can count on us to do the right thing.”

The news follows a difficult year for the fast food giant.

In July, the Chicago-based chain reported its first decline in quarterly same-store sales in about four years, as customers stayed away for reasons ranging from rising inflation to the brand’s perceived support for Israel during the Gaza war.

Chief executive officer Chris Kempczinski warned earlier this month that 2025 looked set to be “another challenging year” for the company as lower-income customers were expected to see their wallets stretched into next year.