McDonald’s has pulled the quarter pounder off its menu in 20% of its stores in the U.S. as the burger chain attempts to mitigate an e. coli outbreak that has already sickened 49 people and killed one. The Centers for Disease Control confirmed the outbreak on Tuesday, confirming that the potentially deadly bacteria was discovered in at least 10 states. Restaurants in “Colorado, Kansas, Utah, Wyoming, and portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, and Oklahoma” have pulled the burger from their menus, according to the agency. The single death so far attributed to the outbreak occurred in Mesa County, Colorado, the county’s public health department announced on Wednesday. At least 26 of the reported cases occurred in Colorado, the agency said. “We fully expect to see more cases,” CDC spokesperson Tom Skinner told Reuters, even though he acknowledged the company has “moved rather quickly” to contain the outbreak. Authorities have not yet pinpointed which ingredient is making people sick, but the Food and Drug Administration said it was potentially the onions served on the burgers. Quarter pounders are typically served with fresh slivered onions—while other menu items, including the Big Mac, are served with diced and cooked onions.