The US Centers for Disease Control and Prevention (CDC) announced on October 22 an investigation into an outbreak of E. coli infections in 10 states across the country. Most people affected by the outbreak have reported eating a Quarter Pounder hamburger at McDonald's before becoming sick. This fast-moving investigation is working to find the source of the contamination in collaboration with McDonald's, the Food and Drug Administration (FDA), the Food Safety and Inspection Service (FSIS), and other public health officials.

According to the CDC food safety alert, Quarter Pounder hamburgers — which are made entirely of fresh onion slices and quarter-pound beef patties — are sourced from McDonald's locations in Colorado, Kansas, Utah, Wyoming and parts of Idaho, Iowa, Missouri, Montana, Nebraska , Nevada, New Mexico and Oklahoma until the investigation confirms the source of the E. coli bacteria.

Initial tracking information from the FDA shows onions are likely a source of contamination, and the agency is working quickly to confirm and determine whether they were used or sold elsewhere. FSIS is also conducting further investigation into hamburger patties as a source of disease.

Taylor Farms is recalling onions as Taco Bell, KFC, Pizza Hut and Burger King are removing them from menus

On October 22, US Foods, a major restaurant supplier, issued a new recall warning for Taylor Farms whole and sliced ​​onions. The recall states: “US Foods Quality Assurance has received notification that Taylor Farms is conducting a recall of onions…due to possible E. coli contamination.”

After the warning was sent, Taco Bell, KFC and Pizza Hut (three fast-food chains under the Yum! Brands umbrella), as well as Burger King, began removing onions from menus at some locations. However, it is currently unclear whether this is directly related to the Taylor Farms recall.

“As we continue to monitor the recently reported E. coli outbreak and out of an abundance of caution, we have proactively removed fresh onions from select Taco Bell, Pizza Hut and KFC restaurants,” said a Yum! representative. Brands told CNBC.

Where is the E. coli outbreak at McDonald's Quarter Pounder?

So far, these states have reported cases of E. coli linked to the outbreak: Colorado, with 26; Nebraska with nine; Wyoming and Utah, both with four; Oregon, Montana, Kansas, Iowa, Wisconsin and Missouri, each with one.

The illnesses began between September 27th and October 11th. Of the 28 people with available information, 10 were hospitalized, including one person who developed a serious condition that can lead to kidney failure. One death was reported in Colorado.

The actual number of people affected is likely much higher and spreads across more states, as outbreaks of this size typically take three to four weeks to be reported.

What to do if you ate a Quarter Pounder and have symptoms of an E. coli infection?

The CDC recommends that you call your doctor if you have eaten a McDonald's Quarter Pounder hamburger and have severe E. coli symptoms. Symptoms include stomach cramps, diarrhea, excessive vomiting, fever over 100°F (39°C), and/or dizziness, which may occur three to four days after ingesting the bacteria. Most people recover on their own without treatment in five to seven days. However, serious complications can occur. Therefore, it is important to consult a professional if you have any concerns.