A food poisoning outbreak tied to McDonald’s Quarter Pounders was caused by a common type of E. coli bacteria that can cause serious illness and death. Particularly vulnerable are young children, older people or those with weakened immune systems.

The CDC has opened an investigation into the fast food giant after tainted Quarter Pounders left one dead, 10 hospitalized and dozens ill.

A preliminary investigation suggested fresh slivered onions that are served raw on the burgers were a likely source of the contamination.