John Snow was a doctor and an anesthesiologist in the early to mid-1800s. During his lifetime, he was known more for his breakthroughs in the field of anesthesiology than for his pioneering studies on the origins of cholera. It says much about his independence and his disregard for social prestige that he sought to prove his unpopular theories about cholera at the height of his career, therefore risking his reputation. His ambitions seem to have been entirely intellectual, rather than social. His indifference to social reputation may have had its roots in his modest background—he was the son of a laborer—as well as in his character. 

Snow is a significant figure to Johnson not only because of his important findings in regard to cholera but because of his methodology. He was both wide-ranging and intensely local in his way of working, drawing on disciplines other than his own (such as statistics) and drawing his conclusions from careful empirical observation. This made him a rarity at a time when many other intellectuals tailored their research to fit their theories, and Johnson suggests that Snow’s methodology is still relevant to us today and may even have helped to usher in the modern age.