Is cancer a contagious disease? In the late nineteenth century this idea, and attending efforts to identify a cancer "germ," inspired fear and ignited controversy. Yet speculation that cancer might be contagious also contained a kernel of hope that the strategies used against infectious diseases, especially vaccination, might be able to subdue this dread disease. Today, nearly one in six cancers are thought to have an infectious cause, but the path to that understanding was twisting and tu...