Our reliance on numbers to understand epidemics can be traced to the development of epidemiology — when medical and scientific authorities had not yet uncovered how microbes caused the spread of infectious disease. Between 1755 and 1866, when epidemiology emerged, medical practitioners believed that environmental factors caused disease. Based on this inaccurate view, they had few effective metrics to understand the origins of epidemics. As such, they counted the number of uninfected and infected patients; the number who contracted a disease and the number who died; they examined those who were hospitalized and those released.
Counting was a way to rationalize infectious disease and to create a narrative about it. For example, during the Crimean War in the 1850s, the nurse and statistician Florence Nightingale witnessed that more British soldiers died once they were admitted to the hospital, but she couldn’t see the germs that were infecting them. What she could see, she counted: the number of healthy and the number of sick soldiers, inside and outside hospitals. By creating a clear analytical assessment, she then observed how the unsanitary conditions within hospitals correlated with alarming mortality rates. According to Nightingale, a “complete system of sanitary statistics in the army” was necessary “to administer the laws of health with that certainty.”
Statistics, and exploring the behaviors behind them, became a key component in epidemiological analysis because that’s all that health experts had — and it helped them craft treatment strategies.
In response to a cholera outbreak in Calcutta, known today as Kolkata, William Twining, a British military doctor there, published an influential comprehensive volume on diseases in 1832. The treatise provided copious detail of hospital attendants who came into close contact with cholera patients and soiled linens but did not become ill. Had the text focused solely on people who became sick, a reader might have been misled about the risk of the disease, or led to look for its causes in the wrong place. With context about the unafflicted, the study offered key evidence that cholera was not transmitted through direct contact.
It was another set of counterexamples two decades later that helped the young science of epidemiology to zero in on the culprit. John Snow, a physician in London, famously found the common denominator among cholera cases in an 1854 outbreak: Those who became sick seemed to all have drunk water from a pump in the center of a poor neighborhood. Cementing his conclusion was the fact that employees at a nearby brewery, which had its own pump, did not contract cholera.