A person has died after eating a McDonald’s Quarter Pounder contaminated with E. coli, according to the U.S. Centers for Disease Control and Prevention (CDC) on October 23. Since the outbreak began on October 22, 49 people across 10 states have fallen ill, and 10 have been hospitalized.

The CDC, along with the FDA, USDA, and state health departments, is investigating the outbreak, which has primarily affected people who ate McDonald’s Quarter Pounders. Most of the cases have been in Colorado and Nebraska, with additional cases in eight other states.

Though the exact cause hasn’t been confirmed, early evidence suggests slivered onions used in the Quarter Pounders might be responsible. These onions come from one supplier serving three distribution centers. McDonald’s has temporarily stopped using these onions in affected states and removed the Quarter Pounder from menus in some areas, including Colorado, Kansas, Utah, and Wyoming.

The USDA is also investigating to rule out ground beef as a possible source of the outbreak.