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McDonald's has temporarily removed quarter pounders and fresh sliced onions from the menu at about a fifth of its U.S. stores due to an outbreak of deadly E. coli poisoning.
So far, the Centers for Disease Control and Prevention has recorded 49 cases of illness in 10 states. Ten cases required hospitalization of patients and one person died.
Most cases of E. coli, a type of bacteria that can cause serious stomach problems, have been recorded in western and midwestern states, according to the CDC.
As health authorities continue to investigate the source of the outbreak, other major food companies have also announced they are removing onions from the United States.
Here's what we know so far.
How big is the epidemic?
So far, the 49 cases of E. coli were largely concentrated in the states of Colorado and Nebraska, the CDC announced Tuesday.
Of the 26 people who became ill in Colorado, one elderly person died – the first death and the only one linked to the outbreak.
The CDC said an infected child is hospitalized with a condition known as hemolytic uremic syndrome (HUS), which can cause kidney failure.
The first confirmed case of the outbreak was reported on September 27, but McDonald's said authorities only informed the company of their concerns late last week.
As of Wednesday, about a dozen people surveyed had identified eating a quarter of a pound before contracting the disease.
The chain's decision to remove patties and onions from the menu affects stores in Colorado, Kansas, Utah, Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana , Nebraska, Nevada, New Mexico and Oklahoma.
Other hamburger products are not affected.
What is the source of the epidemic?
Investigators are considering the possibility that onions, a popular topping for McDonald's quarter-pounders, could be the source of the contamination.
The CDC and FDA said Tuesday that they have not yet ruled out that the patties themselves may be responsible.
The chain agrees with that assessment, although the company said its burgers are cooked to 175 degrees, which is higher than the 160 degree level needed to kill E. coli bacteria.
The cases involve purchases at multiple stores, making it unlikely that food preparation is to blame, McDonald's said.
The company said the affected stores used multiple suppliers for the beef patties but shared a single onion supplier, identified as California-based Taylor Farms, one of the world's largest vegetable processors. .
Taylor Farms, which works with major food suppliers such as US Foods, issued its own recall of certain batches of onions out of an abundance of caution.
Taylor Farms said in a statement provided to CBS News that it had conducted tests on “raw and finished” onion products and “found no traces of E. coli.” coli”.
On Thursday, fast food company Yum! Brands said it was monitoring the outbreak and decided to proactively remove fresh onions from “select Taco Bell, Pizza Hut and KFC restaurants” in the United States.
The company declined to specify how many sites were affected by this decision.
What is E. coli poisoning and what are the symptoms?
E. coli is a diverse group of bacteria that normally live in the intestines of humans and animals.
Although many are harmless, some produce toxins that can make humans very ill.
Symptoms include severe and sometimes bloody diarrhea, stomach cramps, vomiting and fever.
Some infections can lead to other, more serious problems, including kidney failure.
Symptoms tend to appear three to nine days after eating contaminated food.
What is the impact on McDonald's?
McDonald's shares opened down 7% on Wednesday, after news of the outbreak became public. Since then, they have regained ground.
The burger giant said this week it was too early to say how damaging it would be to sales.
It said it believes it has eliminated the problem from its supply chain and aims to return the quarter pounders to affected states in the coming weeks.
But the problem arose when McDonald's was already on the defensive as customers cut back on spending on fast food.
Changing consumer spending has forced McDonald's and other chains to rely heavily on discounts and other promotions.
