Shares of Chipotle Mexican Grill (NYSE: CMG) rallied after the company reported second-quarter earnings on Tuesday that beat earnings expectations, but prospective investors in the burrito maker should nevertheless tread carefully.While its shares are trading at their lowest price since 2013, they could fall further from here, depending in no small part on the direction of its same-store sales in response to the latest food-borne-illness outbreak at the chain.Data source: Chipotle Mexican Grill.