That’s Yiyin Erin Chen, assistant professor of biology at MIT. Chen and some colleagues at Stanford had eliminated an aggressive form of skin cancer from mice — simply by rubbing a swab of bacteria on their furry little heads. The swab contained genetically engineered forms of a bacteria called Staphylococcus epidermidis. Commonly found on human skin, Staphylococcus epidermidis is an innocuous enough passenger, minding its own business without triggering an immune response. But when it was engineered to carry a tumor antigen, S. epidermidis was sparked an aggressive immune reaction — strong enough to wipe out skin cancer tumors in...