A 10-state E. coli outbreak linked to McDonald’s Quarter Pounder hamburgers has killed one person in Colorado and sickened at least 26 others in the state, the Centers for Disease Control and Prevention announced Tuesday.

The CDC reported that 49 people nationwide became ill and 10 required hospital treatment, including one child who suffered serious kidney complications. All of the sick people said they had eaten at McDonald’s and “most” said they had eaten Quarter Pounders, the federal health agency said.

The Colorado Department of Public Health and Environment said the person who died was “elderly” and had underlying health conditions, but did not name the victim.

Neither state nor federal officials reported which McDonald’s restaurants in Colorado served the people sick with E. coli.

People over 65 and children under five are at higher risk of serious illness from E. coli, a type of bacteria that causes vomiting, severe stomach cramps and diarrhea, often with blood. Most people recover without treatment, but some develop severe dehydration or kidney damage.

In addition to Colorado, McDonald’s diners also reported getting sick between September 27 and October 11 in Oregon, Montana, Utah, Wyoming, Nebraska, Kansas, Iowa, Missouri and Wisconsin.

Colorado has the most cases with 26, followed by Nebraska with nine.

“This outbreak may not be limited to states with known illnesses, and the actual number of people sickened is likely much higher than the reported number,” CDC officials said on the agency’s outbreak page. “This is because many people recover without medical care and are not tested for E. coli.”

The state health department said the U.S. Food and Drug Administration believes “freshly sliced ​​onions” used in Quarter Pounders – and not other McDonald’s menu items – may be the culprit, although the U.S. Department of Agriculture is still investigating. whether this is the case with beef patties could be the source of the outbreak.

McDonald’s said in a statement posted on the fast-food chain’s corporate website: “A subset of illnesses may be associated with onion slivers used in the Quarter Pounder and purchased from a single supplier serving three distribution centers.”

Cesar Piña, McDonald’s chief supply chain officer for North America, said in a statement that the company had removed the onions from the supplier to the affected areas and temporarily removed the Quarter Pounder from the menu in Colorado and the other states where customers were sick , in addition to parts of New Mexico, Nevada, Oklahoma and Idaho.

The company is working with the CDC to safely reintroduce all items, he said.

“We are working closely with our suppliers to replenish supplies for the Quarter Pounder in the coming weeks (timing varies depending on local markets),” he said in a statement. “In the meantime, all other menu items, including other beef products (including cheeseburgers, hamburgers, Big Mac, McDouble and the double cheeseburger), are unaffected and are available.”