The CDC, FDA, USDA, and public health officials are investigating an outbreak of E. coli infections linked to McDonald’s Quarter Pounder hamburgers. Most of the illnesses have been reported in Colorado and Nebraska. Possible Source of Contamination While the exact source of contamination is still under investigation, McDonald’s has proactively stopped using fresh slivered onions and quarter pound beef patties in several states. These ingredients are primarily used in Quarter Pounders. Investigators are working to confirm the contaminated ingredient and whether it was distributed to other resta…