The Centers for Disease Control and Prevention is warning of an E. coli outbreak linked with McDonald's Quarter Pounders.

The agency says one person is dead and 10 have been hospitalized due to the outbreak. Forty-nine people have been sickened in 10 states, the CDC said, with most illnesses in Colorado and Nebraska.

"This is a fast-moving outbreak investigation," the CDC said in a bulletin. "Most sick people are reporting eating Quarter Pounder hamburgers from McDonald's and investigators are working quickly to confirm which food ingredient is contaminated. McDonald’s has pulled ingredients for these burgers, and they won’t be available for sale in some states."

The CDC says McDonald's will stop serving the burgers termporarily in some states while it looks into the matter, and investigators are trying to figure out which ingredient is making people sick and if it was sent to other restaurants or stores.

McDonald's said in a statement that initial findings pointed to slivered onions used in the Quarter Pounder which are sourced by a single supplier that serves three distribution centers. The CDC said McDonald's reported to them that "it has stopped using fresh slivered onions and quarter pound beef patties in several states."

"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," McDonald's North America Chief Supply Chain Officer Cesar Piña said in a statement, adding that "out of an abundance of caution," the burger will temporarily not be sold in Colorado, Kansas, Utah and Wyoming, and parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma.

"We take food safety extremely seriously and it’s the right thing to do," Piña said, adding that they "are working in close partnership with our suppliers to replenish supply for the Quarter Pounder in the coming weeks" and assured customers that the chain's other beef products, like the Big Mac, are "unaffected and available."

In a video message, McDonald's USA President Joe Erlinger said the decision to pull the burger was "not one we take lightly," and was made in "close consultation" with the CDC, noting that the "majority of states and the majority of menu items" have not been impacted. 

The CDC urged anyone who has eaten a Quarter Pounder and has severe E. coli symptoms -- like a fever higher than 102°F, diarrhea for more than three days that isn't improving, excessive vomiting or signs of dehydration -- to call their health care provider immediately.

Symptoms of E. coli usually begin 3-4 days after ingesting the bacteria, the CDC said, with most people recovering after 5-7 days with treatment.