Cancer cells spread due to their insidious ability to bypass the body's immune system. To fight them, oncologists have historically turned to toxic drugs that kill a cancer's dividing cells. But over the past decade, researchers have been figuring out how to trick the immune system into attacking tumors. In late 2015, the FDA approved Amgen's IMLYGIC, a genetically engineered virus that might trigger the immune system to kill cancer. For now, the drug is only for melanoma, but it's a harbinger of a major shift in the fight against cancer. University of Pennsylvania's Carl June, a leader in immunotherapy research, explains how IMLYGIC, and other drugs like it, could change how cancer is treated.