An E. coli outbreak linked to McDonald's Quarter Pounder hamburgers has prompted a recall of yellow onions by Taylor Farms, believed to be a contributing factor. The outbreak has resulted in 49 illnesses and one death across 10 U.S. states, leading McDonald's to pull the affected burgers and onions from numerous locations. Other fast-food chains like Taco Bell, Pizza Hut, and KFC, under Yum! Brands, have also removed fresh onions from their menus as a precautionary measure, despite no direct link to their products. Taylor Farms' recall has affected other businesses like Illegal Pete's, which also suspended onion use pending further investigation. Health authorities are still determining whether the E. coli contamination originated from the onions or potentially undercooked beef patties, which also form part of the Quarter Pounder's ingredients. The Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) are continuing to investigate the outbreak's source.