The McDonald's E. Coli outbreak is over

After weeks of investigation, public health officials concluded in late October that contaminated onions were the source of the illness that affected 104 people across 14 states, resulted in one death, and even led to a lawsuit.

The outbreak was traced back to McDonald’s after interviews with those who got sick revealed that nearly all of them had eaten at the fast food chain. Many reported eating menu items with slivered onions, which led investigators to the contaminated supply.

In late October, McDonald’s temporarily removed its Quarter Pounders from about one-fifth of its more than 14,000 U.S. locations as the CDC examined whether the outbreak was linked to the chain’s beef supply or its onions. The investigation soon pointed to onion supplier Taylor Farms in Colorado.

Soon after, a big onion recall was issued, prompting U.S. Foods (USFD), the largest food service supplier in the U.S. to advise restaurants to stop using onions from Taylor Farms and to “destroy” any remaining stock. Fast food chains like Burger King and Taco Bell also pulled slivered onions from their menus.

At the time, public health inspectors from the Food and Drug Administration (FDA), the U.S. Department of Agriculture (USDA), and CDC were collecting data to determine the cause. The USDA was said it was examining the beef patties, while the FDA zeroed in on the onions.

Though the case is officially closed, the incident serves as a reminder of the critical importance of food safety, especially in the wake of outbreaks at other chains like Chipotle $CMG (CMG), which caused long-term sales declines.

It remains to be seen what impact the outbreak will have on McDonald’s sales. Executives did not mention the incident during the Oct. 29 earnings call. In November, McDonald’s said it planned to spend $100 million on E. Coli recovery efforts.