(NEW YORK) -- Dozens of people across the U.S. have contracted E. coli traced to ingredients in McDonald's Quarter Pounders, the Centers for Disease Control and Prevention (CDC) said Tuesday.

As a result, McDonald's announced it has proactively removed two ingredients from stores across two impacted regions, but the company's leadership team has assured that a majority of other menu items are not impacted, according to the CDC investigation.

What to know about McDonald's E. coli outbreak

McDonald's says either fresh, slivered onions or beef patties used for the Quarter Pounder may be behind the outbreak.

If onions are found to be the source of the E. coli outbreak, it would be the first time onions have ever been a carrier for this particular strain of the bacteria, company spokespeople said on Wednesday.

The spokespeople also reiterated that onions used in the Quarter Pounders, as identified by CDC tracing data, came from suppliers that also test for E. coli.

McDonald's uses multiple suppliers for the slivered onions in the area where the outbreak cluster occurred, but all of the product has been retrieved from stock and it will not be supplying more onions until the investigation is completed.

States impacted by McDonald's Quarter Pounder E. coli outbreak

So far, one person has died and a total of 49 people from 10 states have fallen ill with a strain known as E. coli O157:H7, which causes a severe intestinal infection in humans.

The person who died was a resident of Mesa County, Colorado, in the western part of the state, according to the county health department.

Most of the cases have been in Colorado, which has 27 reported cases, and Nebraska, which has nine reported cases, according to the CDC.

The other states are Iowa, Kansas, Missouri, Montana, Oregon, Utah, Wisconsin and Wyoming.

Ten people have been hospitalized, including a child with complications from hemolytic uremic syndrome (HUS) -- a disease that affects the kidneys.

Number of those sickened in outbreak expected to grow

The number of confirmed cases related to the McDonald's E. coli outbreak could grow.

The CDC says the investigation is "fast-moving," with new cases being reported on a rolling basis as the investigation connects people reported being ill and the specific strain of E. coli in this outbreak.

McDonald's stopped selling the Quarter Pounders in the select states just recently, so there may be more cases that develop. The typical onset of symptoms is 3-4 days, and recently ill people may not be part of the current reporting as it usually takes 3-4 weeks to determine if a sick person is part of an outbreak.

The true number of sick people in this outbreak is likely much higher than the number reported, because many people recover without medical care and are not tested for E. coli.

Investigation into McDonald's ingredients potentially responsible for illness

Every patient interviewed by the CDC said they ate at McDonald's before falling ill and most said they ate Quarter Pounder hamburgers, specifically.

It's not clear which ingredient is responsible for the illnesses, but CDC investigators are focused on two ingredients: fresh, slivered onions and fresh beef patties.

"Heat destroys things like bacteria and viruses but in certain cases, especially when you're producing massive amounts of food -- and you have products that are not going to undergo heat, for example, vegetables -- that leads to an opportunity for contamination," ABC News medical correspondent Dr. Darien Sutton explained.

McDonald's told the CDC it has removed slivered onions and beef patties used for Quarter Pounder hamburgers from stores in the states where cases have been reported, the federal health agency said. As a result, the CDC said Quarter Pounders won't be available for sale in some states.

In a statement on Tuesday evening, McDonald's North America Chief Supply Chain Officer Cesar Piña said that the fast food chain believes the outbreak is linked to slivered onions "used in the Quarter Pounder and sourced by a single supplier that serves three distribution centers."

"As a result, and in line with our safety protocols, all local restaurants have been instructed to remove this product from their supply and we have paused the distribution of all slivered onions in the impacted area," the statement read, in part. "We will continue to work with the Centers for Disease Control and Prevention and are committed to providing timely updates as we restore our full menu."

"The decision to do this is not one we take lightly, and it was made in close consultation with the CDC," Joe Erlinger, President of McDonald's USA, said in a statement. "It's important to note that the majority of states and the majority of menu items are not affected."

Slivered onions may be linked to strain of E. coli for 1st time

McDonald's said it is actively working with the CDC, the U.S. Food and Drug Administration (FDA) and other health authorities as they investigate.

During the two-week period in which cases were reported, from Sept. 27 to Oct. 11, the fast food chain sells about 1 million Quarter Pounders, making tracing the outbreak complicated, according to company spokespeople.

Because contamination was not linked to a single restaurant, it likely means the outbreak was not caused by the preparation and handling of the food and was likely higher up the chain, the spokespeople said.

If beef patties are behind the contamination, it would mean restaurants failed to cook the food properly to the standard of 175F. However, the slivered onions topping, which are uncooked, are processed at a facility and then sent to McDonald's.

The company spokespeople said if the slivered onions are identified as the culprit, it would be the first time onions would be a carrier for the O157:H7 strain of E. coli.

McDonald's said they believe only Quarter Pounders are affected and that case numbers would be much higher if other products were associated with the outbreak.

Currently, one-fifth of McDonald's stores are not selling Quarter Pounders. Local restaurants customers know if their location is not serving Quarter Pounders, otherwise the product is available, the spokespeople said.

What to know about E. coli symptoms, recovery

Although most E. coli bacteria are harmless and are part of a healthy intestinal tract, some strains of the bacteria can make people sick, according to the CDC.

E. coli symptoms often begin three to four days after ingesting the bacteria and include severe stomach cramps, diarrhea that may be bloody and vomiting.

Most people recover on their own within five to seven days, but some people may develop HUS and require hospitalization.

The CDC urges those who are experiencing severe E. coli symptoms and recently ate a McDonald's Quarter Pounder to contact their health care provider immediately.