McDonald's E. coli outbreak kills 1 person in the U.S. How can onions get contaminated? Plus 7 other foods commonly linked to food poisoning

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An E. coli outbreak at McDonald's locations in some parts of the United States has caused at least one death and 10 hospitalizations. Currently, 49 people in states like Colorado and Nebraska are experiencing illness after eating Quarter Pounders at the fast food chain, according to the Centers for Disease Control and Prevention (CDC).

It's not yet known which specific ingredient in the hamburgers caused the infections, as investigators work to confirm the potential culprit and if it went to other restaurants or stores. However, initial investigations suggest the slivered onions served on the hamburgers are a "likely source of contamination," according to the CDC, which launched its investigation on Tuesday.

McDonald's said it has stopped using the onions and has pulled Quarter Pounders from its menus in Colorado, Kansas, Utah, Wyoming, Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma. According to a statement from McDonald's, all other menu items — including beef products such as the Cheeseburger, Hamburger, Big Mac, McDouble and the Double Cheeseburger — are "unaffected and available."

While 49 people are currently reported as being sick, the true number of infections is likely higher. That's because it typically takes three to four weeks to determine if a sick person is part of an outbreak, according to the CDC. Moreover, some people will recover from an infection without medical attention or testing for E. coli.

E. coli, or Escherichia coli, bacteria is typically found in the intestines of animals and humans. Most strains of E. coli are not dangerous, but certain ones — like O157:H7 in this case — can cause a person to get sick with a severe intestinal infection. Read on to learn more about E. coli infections, how onions can become contaminated with the bacteria and which other foods are most commonly linked to food poisoning.

Signs and symptoms of an E. coli infection

The most common symptoms of an infection with Shiga toxin-producing E. coli include severe stomach cramps, diarrhea that's often bloody and vomiting. These symptoms typically begin three to four days after swallowing the bacteria, and most people recover after five to seven days without treatment.

The CDC noted some people may develop serious kidney problems with their infection. If someone gets hemolytic uremic syndrome, also called HUS, they'll likely need to be hospitalized.

What to do if you have severe E. coli infection symptoms

If you think you have severe symptoms of an E. coli infection, it's best to call your health-care provider. Specifically, those severe symptoms may include:

• Diarrhea and a fever higher than 102°F

  • Diarrhea for more than 3 days that is not improving

  • Bloody diarrhea

• So much vomiting where you can't keep liquids down

• Signs of dehydration, such as not peeing much as well as dry mouth and throat

Moreover, you can still spread E. coli even if you don't show symptoms. According to the Public Health Agency of Canada, you should avoid preparing food for other people if you think you're sick with an E. coli infection or another gastrointestinal illness.

Most common foods linked to E. coli

1. Onions

Like other vegetables, there are multiple reasons why onions may become contaminated with E. coli. For one, the bacteria may linger on the surface or interior of the onion if the water used for irrigation is already contaminated.

However, onions can also get E. coli from other sources. Those might include cross-contamination from food handlers with poor hygiene or from animals that simply fly over fields or wander through crops.

While cooking onions with proper heat can kill E. coli, serving dishes with raw ingredients skips this step.

2. Raw meat

One of the most common foods that can be contaminated with E. coli is ground beef, according to experts. However, it can affect other types of meat as well.

Siyun Wang, an associate professor of food safety engineering at the University of British Columbia, previously told Yahoo Canada that cooking meat thoroughly before eating it is essential: "Raw beef is still one of the riskiest products in terms of E. coli contaminations, but [many] people have learned to cook it properly. So that's why we're not seeing as many outbreaks associated with ground beef anymore."

3. Leafy greens

Lettuce is a vegetable that has been linked to a growing number of outbreaks in North America. The leafy green vegetable can become contaminated with E. coli in the field by soil, water, animals or improperly composted manure. It can even become contaminated after its harvest or in the grocery store.

Wang noted we see outbreaks linked to leafy greens because there's typically no cooking involved with this vegetable. The Canadian food safety expert recommended washing lettuce under running water for an extended amount of time to clean it properly.

4. Sprouts

Sprouts can get contaminated with E. coli in the environment, during packaging or during preparation. Those might include alfafa and mung bean sprouts.

Health Canada recommended only buying refrigerated sprouts. Moreover, you should not buy sprouts if they look dark or smell musty, and you should cook sprouts before eating them.

5. Unpasteurized milk

E. coli can also affect unpasteurized milk. This happens when the bacteria spreads from the cow's udders to its milk.

Experts recommend checking the label of the milk you're buying to ensure it says "pasteurized." That label means the milk has been heated to destroy bacteria.

6. Unpasteurized cheese

Cheese can be another concern when it comes to E. coli.

"Cheesemakers have to follow a very strict process in terms of aging the cheese and that's why we won't find raw milk cheese or soft cheese in Canada outside of Quebec," Wang explained. "The aging process does not 100 per cent kill E. coli O57 according to some studies. That's why occasionally we will see E. coli cases."

7. Unpasteurized juice and cider

Unpasteurized fruit juices and ciders are also at risk of becoming contaminated with viruses and bacteria. Fruits used to make these types of beverages can get contaminated at the farm they are grown on, while they're being picked and processed, or while they are being transported.

8. Flour

Flour is another raw food that can become contaminated with E. coli. Since flour comes from grain grown in fields, the grain may get contaminated with bacteria through the soil, water or animal waste. It's advised to cook food with flour before eating it and wash your hands after handling flour.

How to prevent getting sick from E. coli

• Cook food to a safe internal temperature: A meat thermometer is a helpful tool to have on hand when cooking meat to ensure it's cooked properly and safe to eat.

• Wash raw fruits and vegetables: Thoroughly wash your produce under running water to remove bacteria before eating it.

• Wash your hands after handling food: It's always a good idea to wash your hands before and after preparing meals to ensure no contamination happens.

• Watch for recalls: If you have a recalled product at home, it's best to return it or throw it out to prevent any possible illnesses.

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