McDonald’s has stopped selling Quarter Pounder hamburgers in about a fifth of its U.S. restaurants as federal health officials conduct an investigation E. coli outbreak That sickened nearly 50 people in 10 states and killed one.

The fast-food chain sold about a million Quarter Pounders during the period the illnesses occurred, a McDonald’s spokesman said Wednesday. The company believed that more people would have become ill if the contamination came from the patties themselves, adding that the onions used in the burgers may be the cause.

The US Centers for Disease Control and Prevention on Tuesday links McDonald’s Quarter Pounder hamburgers led to the outbreak that sickened people in Colorado, Nebraska and other mountain and western states. Most of those who fell ill said they ate Quarter Pounders from McDonald’s, and investigators are working to determine which food ingredient is contaminated, according to the CDC.

What could have caused the E. coli outbreak?

Cesar Piña, the company’s chief supply chain officer for North America, said in a statement Tuesday that the company’s initial investigation suggests some of the E. coli illnesses may be linked to the onion slivers used in the Quarter Pounder.

According to a company spokesperson, McDonald’s burgers are cooked at 175 degrees, which is above the 160 degree mark required to kill the E. coli bacteria.

However, the chopped onions used as a topping on Quarter Pounder burgers sold in the affected region were uncooked and came from a single supplier.

“If this is the source, it would be the first time that onions have been a carrier of this strain of E. coli,” the spokesman said.

McDonald’s has removed onion slices and quarter-pound beef patties from its locations in Colorado, Kansas, Utah, Wyoming and parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma.

Meanwhile, California-based fruit and vegetable giant Taylor Farms said in a statement to CBS News Colorado on Wednesday evening that it had conducted tests on “raw and finished products” and “found no traces of E. coli.”

However, the company said that “out of an abundance of caution, Taylor Farms Colorado has withdrawn yellow onions produced at our Colorado facility from the market.”

Taylor Farms did not say in their statement whether they delivered to any of the affected McDonald’s locations.

To date, a total of 49 E. Coil cases associated with Quarter Pounders have been reported. Most of the illnesses occurred in Colorado and Nebraska, with 26 becoming ill in Colorado and Nebraska and nine becoming ill in Nebraska, the CDC said. Of the 26 people who fell ill in Colorado, one older adult died, the CDC said. Health officials in Utah and Wyoming each reported four illnesses. Other states reporting illnesses include Iowa, Kansas, Missouri, Oregon and Wisconsin.

At least 10 people were hospitalized. The illnesses began in late September and the most recent incident occurred on October 11th.

A child is hospitalized with hemolytic uremic syndrome (HUS), a serious condition that can lead to kidney failure, according to the CDC.

What you should know about E. Coli

It typically takes three to four weeks to determine whether a sick person is part of an outbreak. Most people infected with the toxin-producing E. coli experience severe stomach cramps, diarrhea, and vomiting. Most recover without treatment in five to seven days.

However, the incubation period of E. coli is only a few days, so the disease would be quickly recognizable to anyone affected, said Donald Schaffner, a food safety expert at Rutgers University. “If you ate these burgers in September and it’s now mid-October and you haven’t gotten sick, you’re probably fine,” he said.

People are advised to call their doctor if they have eaten a McDonald’s Quarter Pounder and are experiencing severe symptoms E. coli symptomsas follows:

• Diarrhea and fever over 38°C
  – Diarrhea for more than 3 days that is not improving
  – Bloody diarrhea
• Vomiting so much that you can’t keep your fluids down
• Signs of dehydration, such as:
  – Reduced urination
  – Dry mouth and throat
  – Feeling dizzy when standing up

Concerns about one of McDonald’s most popular menu items come as fast-food chains try to lure consumers still reeling from high food prices. The company this summer launched a $5 meal to attract lower-income customers.

McDonald’s shares fell more than 5% just before the close of trading on Wednesday.

The outbreak is not the first to be linked to the Golden Arches. In 2018 there was an outbreak of intestinal disease McDonald’s salads have sickened more than 500 people in more than a dozen states.

contributed to this report.