In October 2024, CDC, FDA, USDA and public health officials in several states announced an investigation of an E. coli O157:H7 outbreak associated with eating Quarter Pounders from McDonald's.

A total of 104 people infected with the outbreak strain of E. coli O157:H7 were identified. Illnesses started on dates ranging from September 12, 2024 to October 21, 2024. Of 98 people with information available, 34 were hospitalized. Four people developed Hemolytic Uremic Syndrome (HUS). One person in Colorado died. This person is not one of those who developed HUS.

Of the 81 people interviewed, 80 (99%) reported eating at McDonald’s. Seventy-five people were able to remember specific menu items they ate at McDonald's. Sixty-three of 75 people (84%) reported a menu item containing fresh, slivered onions. At least 7 people ate at a McDonald's while traveling to other states.

On October 22, the Colorado Department of Agriculture (CDA) laboratories analyzed subsamples from all the lots of McDonald's brand fresh and frozen beef patties collected from various McDonald's locations in Colorado and found them to be negative for E. coli.

Preliminary traceback and distribution information reviewed by FDA showed that slivered onions served on Quarter Pounders were a likely source of contamination. Taylor Farms, the supplier of slivered onions for affected McDonald's locations initiated a recall of yellow onions.

On December 3, 2024, the CDC closed the investigation.