Six years ago, the writer Mary Elizabeth Williams got a terrifying diagnosis. She had melanoma, a deadly skin cancer. It eventually spread to stage 4, and the prognosis was terminal. As a last-ditch effort, Williams joined a clinical trial for a still-novel treatment called immunotherapy. And today she is not only alive, but she’s been in complete remission for almost five years. For Williams, immunotherapy has been nothing short of miraculous, but she’s the exception. Most cancers, so far, don’t respond to these new experimental treatments.