The Emperor of All Maladies: A Biography of Cancer

by Siddhartha Mukherjee

Indeed, like many of his epidemiologist peers, Graham was becoming exasperated with the exaggerated scrutiny of the word cause. That word, he believed, had outlived its original utility and turned into a liability. In 1884, the microbiologist Robert Koch had stipulated that for an agent to be defined as the “cause” of a disease, it would need to fulfill at least three criteria. The causal agent had to be present in diseased animals; it had to be isolated from diseased animals; and it had to be capable of transmitting the disease when introduced into a secondary host. But Koch’s postulates had arisen, crucially, from the study of infectious diseases and infectious agents; they could not simply be “repurposed” for many noninfectious diseases. In lung cancer, for instance, it would be absurd to imagine a carcinogen being isolated from a cancerous lung after months, or years, of the original exposure. Transmission studies in mice were bound to be equally frustrating. As Bradford Hill argued, “We may subject mice, or other laboratory animals, to such an atmosphere of tobacco smoke that they can—like the old man in the fairy story—neither sleep nor slumber; they can neither breed nor eat. And lung cancers may or may not develop to a significant degree. What then?”