McDonald's (MCD) is the worst Dow Jones stock Wednesday after the Centers for Disease Control and Prevention (CDC) announced it is investigating an E. coli outbreak linked to the fast-food chain's Quarter Pounder hamburgers."This is a fast-moving outbreak investigation," the CDC said, adding that most of those impacted reported eating the restaurant's Quarter Pounders. Investigators are "working quickly" to identify which ingredient is contaminated. "McDonald's has pulled ingredients for these burgers, and ...