Could this be a good buying opportunity?McDonald's (MCD -1.18%) shares took a hit after the company's popular quarter pounders were linked to an E. coli outbreak in several Western and Midwest states. The U.S. Department of Agriculture believes that the onions used in the burgers were the likely source of the contamination.The contamination has led to around 50 people falling ill and one death thus far. McDonald's, which has a strong history of food safety, nonetheless vowed to improve its safety protocols. ...