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But many epidemiologists argued that such cause-effect relationships could only be established for infectious diseases, where there was a known pathogen and a known carrier (called a vector) for a disease—the mosquito for malaria or the tsetse fly for sleeping sickness. Chronic, noninfectious diseases such as cancer and diabetes were too complex and too variable to be associated with single vectors or causes, let alone “preventable” causes. The notion that a chronic disease such as lung cancer might have a “carrier” of its own sort, to be gilded and hung like an epidemiological trophy on one of those balconies, was dismissed as nonsense.

Excerpted from page 245 of ‘The Emperor of All Maladies: A biography of Cancer’ by Siddharth Mukherjee