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An E. coli outbreak linked to McDonald’s Quarter Pounders has led to at least 49 illnesses in ten states, including one death.

Here’s what we know.

The U.S. Centers for Disease Control and Prevention issued a food safety alert Tuesday in which dozens of people reported eating the Quarter Pounder sandwich at McDonald’s before becoming ill.

No specific ingredient has yet been confirmed as the cause of the outbreak, but the U.S. Food and Drug Administration says the sliced ​​onions or beef patties on Quarter Pounder sandwiches are the likely source of contamination.

McDonald’s has stopped using its onion and quarter-pound beef patties in several states, including Colorado, Kansas, Utah and Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma, while it research continues, the U.S. Centers for Disease Control and Prevention said.

According to the agency, the beef patties are only used for the Quarter Pounders, and the chopped onions are primarily used for the Quarter Pounder and not for other items. Diced onions and other types of beef patties used at McDonald’s are not involved in this outbreak, the FDA said.

Escherichia coli, or E. coli, is a common bacteria, but certain types can make you sick. Infections can occur after ingesting the bacteria, often after eating contaminated food or water. It can also spread from person to person through poor bathroom hygiene.

People with E. coli infections may have symptoms such as severe stomach cramps, diarrhea, fever and vomiting. Symptoms of infection usually begin three or four days after ingesting the bacteria.

Although most people who become ill recover within a week without treatment, others may develop serious kidney problems and require hospitalization. According to the CDC, seniors, children under age 5, and people with weakened immune systems are at the highest risk for infection.

E. coli infections resulting from the McDonald’s outbreak have led to at least ten hospitalizations, including a child with hemolytic uremic syndrome, a serious complication that can arise from an E. coli infection. One elderly person has died.

According to the CDC, most of the illnesses linked to the McDonald’s outbreak are in Colorado and Nebraska, but the agency notes that the outbreak may extend beyond those states. Illnesses have also been reported in Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.

The CDC recommends people call their doctor if they have recently eaten a McDonald’s Quarter Pounder and are experiencing severe E. coli symptoms, such as fever higher than 102 and diarrhea, especially bloody diarrhea or diarrhea that has not improved in three days, vomiting that limits fluid intake or shows signs of dehydration.

It can take weeks to determine whether an illness is part of an outbreak, but the CDC said the investigation into the McDonald’s outbreak is “rapidly evolving.”

The agency said the outbreak is expected to grow, with new cases reported “on a rolling basis” as scientists can draw genetic links between the outbreak strain and the bacteria that cause human infections.

According to CDC data, the onset of illnesses linked to the outbreak was reported from September 27 to October 11. But the start date of the outbreak is also likely to shift as past cases come to light, an official with knowledge of the investigation said.

Outbreaks like this can also subside quickly after the contaminated food or ingredient is removed from the market. That requires a thorough investigation to ensure that all possible distribution channels have been exposed and stopped.

The president of McDonald’s USA said it is safe to eat at McDonald’s and that the affected ingredients are likely not in the restaurant supply chain.

“We are confident that you can go to McDonald’s and enjoy our classics” without getting sick, McDonald’s U.S. President Joe Erlinger said Wednesday on NBC’s “Today Show.”

Quarter Pounder sandwiches will be temporarily removed from McDonald’s menu in some states while the company makes some offering changes, according to the CDC.

Erlinger said he believes that “if there has been contaminated product in our supply chain, it has most likely already passed through that supply chain,” but he acknowledged that the number of reported illnesses could rise as the CDC investigates and tracks cases.

“Our top priority is to serve customers safely in every restaurant, every day,” McDonald’s said in a statement Tuesday.

A fact sheet from the company highlights the food safety protocols it says are in place, including daily temperature checks and hourly hand washing for employees. It also notes that Quarter Pounders are prepared to order at temperatures higher than the FDA code of best practices.

CDC data released this summer shows foodborne illness is on the rise in the US. A few diseases, including E. coli infections, are well above federal goals for reducing foodborne illness.

In 2023, there were more than five E. coli diseases for every 100,000 people in the US – a 25% increase from five years earlier and about 40% higher than targets.

Campylobacter bacteria is the most common pathogen causing foodborne illness, with more than 19 illnesses per 100,000 people – a rate that is 22% higher than five years ago and twice as high as federal goals. Infections from this bacteria are most commonly caused by eating raw or undercooked poultry, according to the CDC, and it can make people sick with diarrhea.

Salmonella infections have not increased in recent years, but the bacteria still caused about 14 illnesses for every 100,000 people in the US in 2023.

Listeria caused about 0.3 illnesses for every 100,000 people in the U.S. in 2023, according to CDC data, but has led to some large, deadly outbreaks this year.

Boar’s Head issued a recall in late July for more than 7.2 million pounds of ready-to-eat liverwurst and some other processed meat products, leading to dozens of hospitalizations and at least nine deaths, and another recall of nearly 12 million pounds of ready-to-eat liverwurst. eatable meat and poultry products made by BrucePac have hit schools, retailers and restaurants across the country.

CNN’s Nadia Kounang, Carma Hassan, Brenda Goodman and Meg Tirrell contributed to this report.