On the Mode of Communication of Cholera

Summary

On the Mode of Communication of Cholera is the book in which the London physician John Snow argued that cholera is spread by contaminated water rather than by bad air. A short first edition appeared in 1849, after the epidemic of 1848–1849; the second edition of 1855, much enlarged, added the evidence from the epidemic of 1854 that made the work famous. Cholera had reached Britain in 1831 and returned in waves, killing tens of thousands. Most doctors believed it spread through miasma, poisonous air rising from filth. Snow reasoned from the disease itself: because cholera begins with symptoms in the gut rather than in the blood or lungs, he thought the cause must be swallowed. He argued that a living agent passed in the waste of the sick reached others through unwashed hands and food and, above all, through drinking water polluted by sewage. The second edition presents two great investigations. In late August and early September 1854, more than five hundred people died within about ten days in a few streets of Soho. Snow found that the deaths clustered around the public pump in Broad Street. People who lived nearby but drew water elsewhere, such as the inmates of a workhouse with its own well and the workers of a brewery who drank beer, largely escaped, while some who lived farther away but preferred the Broad Street water died. At his urging the parish authorities removed the pump handle. Snow's map of the deaths became one of the most famous diagrams in medicine. The second investigation compared households in south London supplied by two water companies, one drawing from the Thames where it was polluted by London's sewage and the other from a cleaner point upriver. Deaths were many times higher among the customers of the first. Snow's conclusions were not widely accepted in his lifetime, but they were confirmed after Robert Koch identified the cholera bacterium in 1883.

Top Lessons

• Reasoning against the crowd

  Snow challenged the medical consensus with careful reasoning and data. Evidence, not majority opinion, settles scientific questions.

• The power of a map

  Snow's map of deaths around the Broad Street pump made a pattern visible at a glance. Good data displays can change minds.

• Natural experiments

  The two water companies served similar neighbourhoods, so comparing them was almost like a controlled experiment. Snow saw an opportunity in ordinary circumstances.

• For the Christian reader

  Cholera struck the poor hardest. Snow's work, carried out in crowded streets among the poor, shows how careful study can serve the common good and the neighbour in need. Concern for clean water echoes the biblical care for the vulnerable.

Verified Quotes

Medical Lens

A founding work of epidemiology showing that cholera is water-borne.

• Broad Street outbreak (1854)

  Mapping deaths around a public pump.

• South London water companies

  Comparison of death rates by water supply.