Contagious cancer: The evolution of a killer

Under ordinary circumstances, cancer is an individuated phenomenon. Its onset is determined partly by genetics, partly by environment, partly by entropy, partly by the remorseless tick-tock of time, and (almost) never by the transmission of some tumorous essence. It arises from within (usually) rather than being imposed from without. It pinpoints single victims (usually) rather than spreading through populations. Cancer might be triggered by a carcinogenic chemical, but it isn’t itself poisoning. It might be triggered by a virus, but it isn’t fundamentally viral. Cancer differs also from heart disease and cirrhosis and the other lethal forms of physiological breakdown; uncontrolled cell reproduction, not organ dilapidation, is the problem.

. . . but the evidence was leading inexorably to a strange and frightening new hypothesis: the cancer itself had somehow become contagious.