The news initially broke Monday night. However, further reports of illness are causing a selloff. In afternoon trading, Chipotle fell almost $30, or 6.34%, to $435.98.
The news comes on National Avocado Day, which Chipotle is celebrating by giving customers who order through the mobile app or online free guacamole.
In 2015, Chipotle had multiple break-outs of food-borne illness that affected multiple locations throughout the U.S. The illnesses were attributed later to poor food safety protocols, which greatly hurt Chipotle's reputation.