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A McDonald's sandwich has been making people sick in the US, according to the Centers for Disease Control and Prevention (CDC).

E. coli, a type of bacteria that can cause serious stomach problems, has been found in McDonald's Quarter Pounder sandwiches, the CDC announced on Tuesday.

So far, the CDC has recorded 49 cases of illness across 10 states. Ten cases resulted in patients being admitted to hospital and one person has died.

Most of the cases were recorded in western and Midwest states, according to the CDC.

The fast-food restaurant is working with investigators to determine which ingredients caused the outbreak, according to a statement from the CDC.

“McDonald’s has pulled ingredients for these burgers, and they won’t be available for sale in some states,” the agency said.

“It is not yet known which specific food ingredient is contaminated,” the CDC added, noting that McDonald's has already “stopped using fresh slivered onions and quarter-pound beef patties in several states”.

The CDC said that the slivered onions are believed to be the likely source of contamination, and investigators with the Food and Drug Administration (FDA) are working to determine if the onions were sold to any other business.

No recalls have been issued yet by the CDC or by other health and food regulators.

The first case was recorded on 27 September, investigators say. Victims have ranged in age from 13 to 88.

Of the 10 people taken to hospital, one person developed hemolytic uremic syndrome, a serious condition that can cause kidney failure.

Another person, who the CDC described as “an older adult in Colorado” died after eating at McDonald’s.

Cases have been reported in Colorado, Iowa, Kansas, Missouri, Montana, Nebraska, Oregon, Utah, Wisconsin and Wyoming.

McDonald's shares fell by about 9% on the New York Stock Exchange after the news broke on Tuesday.

In a statement, McDonald's said that a preliminary investigation found "that a subset of illnesses may be linked to slivered onions used in the Quarter Pounder and sourced by a single supplier that serves three distribution centers".

The Chicago-based company added that it has instructed all local restaurants "to remove this product from their supply" and have paused shipments of slivered onions to the region.

The sandwich is also being temporarily removed from the menu in several states, the company said, adding: "We take food safety extremely seriously and it’s the right thing to do."

Other beef products remain on the menu, McDonald's USA President Joe Erlinger said in a video message.

"At McDonald's, you can count on us to do the right thing," he said.

E. coli are a diverse group of bacteria that normally live in the intestines of humans and animals.

Although many are harmless, some produce toxins that can make you sick.

Symptoms include severe and sometimes bloody diarrhoea, stomach cramps, vomiting and fever.

It usually takes a few days after being infected for symptoms to show.

This is not the first E. coli outbreak to affect McDonald's in recent years.

In 2022, six children in Alabama were sickened with E. coli after eating chicken McNuggets.

Four children were admitted to hospital. Health inspectors later visited the affected restaurant and found several violations, including improper hand-washing and a lack of gloves.