By Deidre McPhillips, CNN

(CNN) — An E. coli outbreak linked to McDonald’s Quarter Pounders has led to at least 49 illnesses across 10 states, including one death.

Here’s what we know.

Quarter Pounders made people sick

The US Centers for Disease Control and Prevention issued a food safety alert Tuesday warning that dozens of people reported eating the Quarter Pounder sandwich at McDonald’s before becoming sick.

A specific ingredient has not been confirmed as the source of the outbreak, but the US Food and Drug Administration says that the slivered onions or beef patties on Quarter Pounder sandwiches are the likely source of contamination.

McDonald’s has taken Quarter Pounders off the menu in about a fifth of its stores. The company has stopped using the onions as well as quarter-pound beef patties in several states – Colorado, Kansas, Utah and Wyoming, as well as portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma – while the investigation continues, the CDC said.

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

E. coli infections can be serious

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

People with E. coli infections may have symptoms including severe stomach cramps, diarrhea, fever and vomiting. Symptoms usually begin three or four days after swallowing the bacteria.

Although most people who become ill recover without treatment within a week, others can develop serious kidney problems and require hospitalization. Seniors, children younger than 5 and people with weakened immune systems are most at risk of infection, according to the CDC.

E. coli infections from the new outbreak have led to at least 10 hospitalizations, the CDC says, including a child who had hemolytic uremic syndrome, a serious complication that can develop from an E. coli infection. One older person has died.

Most of the illnesses related to the outbreak are in Colorado and Nebraska, according to the CDC, but the agency notes that the outbreak may go beyond those states. Illnesses have also been reported in Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.

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

A ‘fast-moving’ investigation

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 “fast-moving.”

The agency said the outbreak is expected grow, with new cases being reported “on a rolling basis” as scientists make genetic linkages between the outbreak strain and the bacteria that is causing human infections.

The onset of illnesses associated with the outbreak have been reported from September 27 to October 11, according to CDC data. But the start date of the outbreak is likely to shift too as cases come to light, according to an official with knowledge of the investigation.

Outbreaks like these can also wind down quickly after the tainted food or ingredient is removed from the market. That requires a thorough investigation to make sure all possible channels of distribution have been uncovered and stopped.

Changes at McDonald’s

The president of McDonald’s USA said that it’s safe to eat at McDonald’s and that affected ingredients are probably out of the supply chain at restaurants.

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

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

Erlinger said he believes that “if there has been contaminated product within our supply chain, it’s very likely worked itself through that supply chain already,” but he acknowledged that the number of illnesses reported may rise as the CDC investigates and traces cases.

“Serving customers safely in every single restaurant, each and every day, is our top priority,” McDonald’s said in a statement Tuesday.

A fact sheet from the company highlights food safety protocols that it says are in place, including daily temperature checks and hourly handwashing for employees. It also notes that Quarter Pounders are cooked to order to temperatures that exceed the FDA’s code on best practices.

Foodborne illness is on the rise

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

There were more than 5 E. coli illnesses for every 100,000 people in the US in 2023, a 25% increase from five years earlier and about 40% higher than target rates.

Campylobacter bacteria is the most common pathogen causing foodborne illness, causing more than 19 illnesses for every 100,000 people – a rate that’s 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 ill with diarrhea.

Salmonella infections haven’t 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 US in 2023, according to data from the CDC, but it has led to a few large, deadly outbreaks this year.

Boar’s Head issued a recall in late July for more than 7.2 million pounds of its ready-to-eat liverwurst and some other deli meat products, amid reports of dozens of hospitalizations and at least 10 deaths, and a recall of nearly 12 million pounds of ready-to-eat meat and poultry items made by BrucePac has affected schools, retailers and restaurants nationwide.

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