McDonald’s shares dropped in extended trading Tuesday after the Centers for Disease Control and Prevention said an E. coli outbreak linked to McDonald’s Quarter Pounder burgers has led to 10 hospitalizations and one death.

The agency said 49 cases have been reported so far in 10 states, with most of the illnesses in Colorado and Nebraska. “Most” sick people reported eating a McDonald’s Quarter Pounder, the CDC added.

Investigators are working to determine which ingredient may be contaminated, and McDonald’s has withdrawn ingredients for the burgers, the CDC said. Quarter Pounder hamburgers will be temporarily unavailable in some states, but the CDC did not clarify which ones. 

McDonald’s shares dropped about 9% in after-hours trading Tuesday.

E. coli refers to a group of bacteria found in the gut of nearly all people and animals. But some strains of the bacteria can cause mild to severe illness if a person eats contaminated food or drinks polluted water.

Symptoms, including stomach cramps, diarrhea and vomiting, usually start three to four days after swallowing the bacteria, according to the CDC. Most people recover without treatment after five to seven days.

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