A Chipotle Mexican Grill restaurant is seen in Washington, DC, December 22, 2015. Chipotle shares tumbled on news that the Centers for Disease Control and Prevention (CDC) is investigating an outbreak of E. coli that may be unrelated to a previous one in November that led to 53 cases in nine states. AFP PHOTO / SAUL LOEB / AFP / SAUL LOEB (Photo credit should read SAUL LOEB/AFP/Getty Images)