McDonald’s Quarter Pounder hamburgers have been linked to an outbreak of E. coli, a bacteria that can make people sick.

The outbreak has killed one person in Colorado, according to the U.S. Centers for Disease Control and Prevention. It has also affected nearly 50 other people in 10 states.

The illnesses may be linked to slivered onions used on Quarter Pounders or the patty itself, but the investigation into the specific contaminated food ingredient is ongoing, the CDC shared in a post Tuesday.

McDonald’s said it has removed Quarter Pounders from a total of 12 states, but that all other menu items are unaffected.

The CDC added the outbreak may not be limited to the states with known illnesses, either.

“The true number of sick people is likely much higher than the number reported. This is because many people recover without medical care and are not tested for E. coli,” the agency wrote.

Symptoms and early signs of E. coli

E. coli symptoms usually start about two to seven days after exposure, CBS News chief medical correspondent Dr. Jon LaPook said on the “CBS Evening News” Tuesday.

Symptoms can include:

• Diarrhea
• Fever
• Abdominal cramping
• Vomiting
• Feeling dehydrated or dizzy

If you ate a Quarter Pounder and have any of these symptoms, contact your health care provider, the CDC advises. 

“Especially if you have fever over 102, you can’t keep liquids down or your symptoms are lasting more than three days,” LaPook said. 

How E. coli is transmitted

People can get infected with E. coli by ingesting it.

“This can happen through contaminated food or water or contact with animals, environments, or other people,” the CDC’s website notes.

The best way to prevent infection is by keeping your hands clean, preparing food safely and drinking safe water, the agency adds.

How severe can E. coli get

“Most E. coli are harmless and are part of a healthy intestinal tract. E. coli help us digest food, produce vitamins, and protect us from harmful germs. But some E. coli can make people sick with diarrhea, urinary tract infections, pneumonia, sepsis, and other illnesses,” the CDC says. 

For example, certain E. coli infections can lead to a serious health condition called hemolytic uremic syndrome, or HUS, which can cause kidney failure, permanent health problems and death, according to the CDC.

Certain people are also at increased risk for E. coli infection, including: 

• Children under 5
• Adults 65 and older
• People with weakened immune systems
• International travelers