A deadly E. coli outbreak linked to McDonald’s has now sickened over 100 people.

The Centers for Disease Control and Prevention said 104 people across 14 states have fallen ill as of Nov. 13.

Slivered onions served on McDonald’s Quarter Pounders were identified as the likely source of the outbreak. Illnesses started between Sept. 12 and Oct. 21.

Of the 98 people with data available, 34 were hospitalized, the CDC said. Four of them developed hemolytic uremic syndrome, which is a serious condition that can lead to kidney failure.

An older adult in Colorado has died as a result of the outbreak.

The CDC presumes there is a higher number of sick people in the outbreak than is being reported.

“The true number of sick people in this outbreak is likely much higher than the number reported, and the outbreak may not be limited to the states with known illnesses. This is because many people recover without medical care and are not tested for E. coli,” the CDC said in a press release. “In addition, recent illnesses may not yet be reported as it usually takes three to four weeks to determine if a sick person is part of an outbreak.”

Amid the outbreak, McDonald’s removed Quarter Pounders from its menu in affected states. It has since brought back the burgers but is serving them without the slivered onions. Additionally, Taylor Farms, a supplier of onions for the fast food chain, issued a voluntary recall.

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Some people are more at risk of E. coli infection than others. They include young children, adults 65 years and older and people with a weakened immune system, according to the CDC.

Signs of E. coli infection include diarrhea and a fever higher than 102 degrees, vomiting and dehydration. In serious cases, kidney failure and death can occur.

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