Customers hungry for a burrito or taco last fall in Washington state or Oregon may have reached the door of their local Chipotle Mexican Grill Inc. restaurant only to find the following sign: “FYI: We are sorry. But we are temporarily closed due to a supply chain issue. We will reopen as soon as possible.”

A “supply chain issue” sounds fairly harmless, but what was actually happening at the Denver-based, fast-casual food company was anything but. Chipotle temporarily shuttered 43 restaurants in the two states because of an E. coli outbreak linked to its restaurants.