Stacy Erholtz, like many, has been inoculated with the measles vaccine, except she received a dose big — enough that it could have served 10 million people. No, this isn’t a precaution against the disease as more measles outbreaks become apparent, but it is actually part of an experimental trial to use the vaccine to fight cancer. Mayo Clinic researchers used this form of “virotherapy,” when you infect cancerous cells and kill them with virus while leaving normal cells untouched, to treat multiple myeloma. The idea of using a virus to destroy cancer goes back several decades, but this study...