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Today we know cholera spreads through water or food contaminated with the bacterium Vibrio cholerae. In 1854 nobody had identified that cause. John Snow, a London physician, thought the disease travelled in water, and he set out to test it with careful counting. This post covers what happened at Broad Street, how Snow built his case, a worked example using his own table, the truth about his famous map, and the method students can borrow.
What happened in Broad Street in 1854?
Snow described it in the second edition of his book, On the Mode of Communication of Cholera (1855): "The most terrible outbreak of cholera which ever occurred in this kingdom, is probably that which took place in Broad Street, Golden Square, and the adjoining streets." Within 250 yards of the corner of Cambridge Street and Broad Street, there were "upwards of five hundred fatal attacks of cholera in ten days." Many residents fled.
How did Snow trace it to one pump?
Snow got permission to see the deaths registered that week and inquired in detail into 83 of them. Nearly all had taken place a short distance from the Broad Street pump. Of ten deaths in houses nearer another pump, five families told him they always sent to Broad Street because they preferred its water, and three of the dead were children who went to school near it. Of the deaths near the pump, he found 61 cases where the person was known to drink its water, constantly or occasionally. In six cases he could get no information, and in six the person had not drunk the pump water.
He then looked at groups that did not get sick, which is what made his case strong:
A workhouse in Poland Street was almost surrounded by houses with cholera deaths, yet only five of its 535 inmates died of cholera. The workhouse had its own well and never sent to Broad Street for water.
A brewery in Broad Street had more than 70 workmen, and none suffered severe cholera. The owner told Snow they drank malt liquor and the brewery had its own deep well.
On the evening of Thursday 7 September 1854, Snow presented his findings to the Board of Guardians of St James's parish. In his words: "In consequence of what I said, the handle of the pump was removed on the following day."
What was his comparison of two water companies?
In several south London districts, two companies supplied water, with "the pipes of both companies going down every street". In 1852 the Lambeth Company had moved its works to Thames Ditton, which Snow says gave it "a supply of water quite free from the sewage of London". The Southwark and Vauxhall Company's water was not. Neighbouring houses got different water, while the people in them lived in much the same conditions.
When residents did not know their supplier, Snow used chemistry. He added silver nitrate solution to the water: Southwark and Vauxhall water, which contained far more common salt, gave a much heavier precipitate of silver chloride than Lambeth water. It is a precipitation reaction: silver ions and chloride ions form insoluble silver chloride. His Table IX gives deaths in the first seven weeks of the 1854 epidemic.
Worked example: deaths per 10,000 houses
From Snow's Table IX:
Southwark and Vauxhall: 40,046 houses, 1,263 deaths.
Lambeth: 26,107 houses, 98 deaths.
Southwark and Vauxhall rate: 1,263 ÷ 40,046 × 10,000 ≈ 315 deaths per 10,000 houses.
Lambeth rate: 98 ÷ 26,107 × 10,000 ≈ 37.5, which Snow printed as 37.
Ratio: 315 ÷ 37 ≈ 8.5.
Snow's conclusion: mortality in houses supplied by Southwark and Vauxhall was "between eight and nine times as great" as in houses supplied by Lambeth.
Common mistakes:
Comparing raw counts. 1,263 against 98 looks like a 13-fold difference, but the two companies supplied different numbers of houses. Always divide by the size of the group.
Mixing up the base. Snow used houses, not people. A rate is only meaningful when you say "per what".
Forgetting other explanations. The comparison works because the two groups lived side by side. Two groups from very different areas would not show the water's effect so cleanly.
Our guide to statistics, mean and variance covers the CBSE tools for summarising data like this.
Did the map find the pump?
Snow's dot map of deaths around Broad Street is famous. But the handle came off on 8 September 1854, and the map appeared in the 1855 second edition. The first edition of On the Mode of Communication of Cholera (1849) had already set out his water theory five years earlier. The map made the argument visible to others; the house-by-house counting did the detective work. Historians have examined how the map's story grew in the retelling, for example in Map-making and myth-making in Broad Street02442-9) in The Lancet (2000).
Why does his method still matter?
The World Health Organization estimates 1.3 to 4.0 million cholera cases and 21,000 to 143,000 deaths worldwide each year, and says access to safe water, basic sanitation and hygiene is essential to prevent it. Snow did not know about the bacterium, but his reasoning pointed to the same fix: keep sewage out of drinking water. For any health concern of your own, see a doctor; this post is about the history of the method.
What can a student take from Snow's way of working?
Go and count. Snow walked the streets himself instead of relying on reports.
Look for the exceptions. The workhouse and the brewery told him as much as the victims.
Compare rates, not totals. That single habit avoids many errors in data questions.
Snow's work sits beside Florence Nightingale's statistics and Louis Pasteur's germ theory in the story of public health. When rate and ratio questions are slowing you down, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Next step: using Snow's figures, work out how many deaths Southwark and Vauxhall's houses would have had at Lambeth's rate, and what that difference means.
