The outbreak has been linked to the Quarter Pounder specifically, with initial speculation that the cause was either the fresh slivered onions (which are primarily used on Quarter Pounders) or the quarter-pound beef patties. (A departure from its other burgers, McDonald’s launched fresh beef for the Quarter Pounder, as opposed to frozen, in 2018.)

In an official statement, McDonald’s speculated that the issue lies with the slivered onions, which are sourced from a single supplier that serves three distribution centers. The Colorado-based Taylor Farms, which supplied the onions, has since issued a recall of its yellow onions. The distributors Sysco and US Foods have both notified their customers to stop using onions from Taylor Farms, which are also sold under the name Cross Valley Farms, Restaurant Business reported. Some Burger King, Denny’s, KFC, Taco Bell, and Pizza Hut locations have also pulled onions from the menu.