As of this writing, the CDC reports 49 cases in 10 states, including one death and 10 hospitalizations, that have been linked to McDonald's.
Though the outbreak has been linked to the Quarter Pounder specifically, health officials have not yet definitively identified the problem ingredient.
In an official statement, McDonald's speculates that the issue lies with the slivered onions, which are sourced from a single supplier that serves three distribution centers.
This continues an alarming uptick in major foodborne illness outbreaks: In recent months, deli meats, other ready-to-eat meat, and frozen waffles have all been affected by listeria contamination.