Competing Against Luck: The Story of Innovation and Customer Choice

by Clayton M. Christensen

To understand how revolutionary Pasteur’s contributions were, consider the previously popular ideas that attempted to explain why people got sick. For nearly two thousand years, the medical profession believed that four different bodily fluids—blood, phlegm, yellow bile, and black bile—dominated the health and moods of people. When they were in harmony, all was right with the world. When they were out of sync, people fell ill or into “bad humor.” The theory was known as humorism. Doctors were never quite certain what caused imbalance among these humors—ideas ranged from seasons to diet to evil spirits. So they experimented by trial and error to restore the necessary harmony of fluids—often with now seemingly barbaric methods such as bloodletting, which at the time was said to remedy hundreds of diseases. Sometimes, people got better. But most of the time, they got worse. And doctors were never sure why. By the nineteenth century, people began to blame disease on “miasmas” or “bad airs” that floated around dangerously. As hare-brained as it sounds today, “miasma theory” was actually an improvement over humorism because it spawned sanitary reforms that had the effect of removing real disease agents—bacteria. For example, in 1854, when cholera gripped London, the miasma explanation inspired massive, state-sponsored clearing of the air by draining cesspools. A physician of the time, John Snow, was able to isolate the pattern of new cholera cases and to conclude that new cases ...