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.

Plot or Argument

• The nature of cholera

  Snow argues from the symptoms that the disease begins in the alimentary canal and is caused by something swallowed.

• Ways of spread

  He traces cases spread by hands, food and close contact, especially among the poor.

• Water and sewage

  He argues that sewage-polluted water spreads cholera most widely.

• The Broad Street outbreak

  He analyses the Soho outbreak of 1854 and its connection to a single pump.

• The South London comparison

  He compares deaths among customers of two water companies.

• Prevention

  He recommends measures such as clean water supplies, sewage separation and hygiene.

Lessons

• Reasoning against the crowd

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

  • What did most doctors believe about cholera, and why did Snow disagree?
  • What makes evidence persuasive enough to overturn a consensus?

• 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.

  • What did the map show?
  • Why were the brewery workers and workhouse residents important evidence?

• 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.

  • Why was the water-company comparison so convincing?
  • Where might a natural experiment be found today?

• 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.

  • How can scientific work be a form of loving one's neighbour?
  • Why do diseases often fall hardest on the poor?

Context

London in the 1840s and 1850s had grown faster than its drainage. Cesspools and sewers emptied into the Thames, from which some water companies drew supplies. Cholera epidemics struck in 1832, 1848–1849 and 1853–1854.

Composition

Snow published a pamphlet of about forty pages in 1849. After the 1854 epidemic he expanded it into a book of more than one hundred and thirty pages, published by John Churchill in London in 1855, with maps of the Broad Street area and of the water-company districts.

Reception

A committee of the parish and the General Board of Health studied the Broad Street outbreak; the Reverend Henry Whitehead, at first sceptical, became convinced and helped identify the first case. Acceptance of the water theory grew after the epidemic of 1866 and Koch's discovery of the bacterium.

Comparisons

• Notes on Nursing: What It Is, and What It Is Not

  Nightingale, a believer in miasma, also stressed pure water and drainage.

• Germ Theory Papers (Pasteur and Lister)

  The germ theory explained the living agent Snow inferred.

• The Contagiousness of Puerperal Fever

  Holmes, like Snow, inferred a mode of transmission from patterns of cases before germs were understood.

• The Plague

  A novel about an epidemic and the people who fight it.

Quotes

For Students

A reading companion for classical Christian high school students. See the student guide.

Reading Guide

Watch for

• How Snow argues from symptoms to the route of infection.
• The evidence from the Broad Street outbreak.
• Exceptions that support his theory.
• The comparison of water companies.

Key terms and names

Miasma

The theory that disease is caused by poisonous air from decaying matter.

Epidemiology

The study of how diseases spread in populations.

Cholera

A bacterial disease causing severe diarrhoea and dehydration, often fatal without treatment.

Natural experiment

A comparison in which circumstances, not the researcher, assign people to different conditions.

Seminar Questions

1. Comprehension What did Snow conclude about how cholera spreads?
2. Comprehension What happened after the pump handle was removed?
3. Interpretation Why were the brewery workers important to Snow's argument?
4. Interpretation Why was the comparison of water companies so persuasive?
5. Interpretation How did Snow's comparison of water companies strengthen the evidence from the Broad Street pump?
6. Big Question How should public officials act when evidence is strong but not yet accepted?

Faith Connections

• Ezekiel 47:9 (Scripture)

  Living water brings life wherever it flows; clean water as a blessing.

• Deuteronomy 23:12–13 (Scripture)

  Ancient Israel's law on sanitation outside the camp.

• Hippocrates, Airs, Waters, Places (Great Conversation)

  The ancient link between water and health.

Logic and Rhetoric

Central argument: Cholera is caused by a living agent swallowed in food or water contaminated by the waste of the sick, so clean water can prevent it.

Rhetorical device: evidence from exceptions. Snow strengthens his case by explaining apparent exceptions, such as brewery workers who drank no pump water.

Debate: Resolved: that public health officials should act on strong evidence before scientific consensus is reached.

Writing Prompts

Essay: Explain how Snow used maps and comparisons to show that cholera spreads through water.

Comparison with Germ Theory Papers (Pasteur and Lister): Compare Snow's inference of a disease agent with Pasteur's laboratory proof of germs.

Medical Lens

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

• Broad Street outbreak (1854)

  Mapping deaths around a public pump.

  Plague Epidemic Public Health Sanitation
• South London water companies

  Comparison of death rates by water supply.

  Public Health Sanitation

Creative Prompts

• Classroom

  Make a simple map of an imaginary outbreak and show how it could reveal the source.

• Essay

  Explain how Snow's reasoning differed from the miasma theory.

• Dialogue

  Write a meeting at which Snow persuades the parish board to remove the pump handle.

Further Reading

• The Ghost Map, Steven Johnson

• Cholera, Chloroform, and the Science of Medicine: A Life of John Snow, Peter Vinten-Johansen and others