Infection and immunity
Infection and immunity

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Infection and immunity

2.5  John Snow’s experiment

In 1854 Snow did not know what was in the water from the Broad Street pump, or if the water was indeed the source of cholera. Vibrio cholerae bacteria were not identified until almost 30 years later in India in 1883, by the pioneering German physician and scientist Dr Robert Koch (1843–1910; Figure 8).

Figure 8  Robert Koch, photographed in 1896 or 1897 at work in his laboratory.

Activity 1  What does Snow’s experiment tell us about the scientific method?

Allow about 30 minutes to work through this activity and answer all the questions.

Despite his ignorance of bacteria, Snow’s approach to identifying and eliminating the source of the 1854 outbreak elegantly illustrates the main features of the scientific method. Take a moment to look back at the definitions in Section 2.1 and then answer the following questions:

Question 1

How does Snow’s starting point for tackling the outbreak illustrate the scientific principles of systematic observation and measurement?


He made an accurate map of the households in his district and recorded the number and location of every death from cholera. He had a fixed, pre-determined ‘system’ for making observations and measurements of the scale and geography of the outbreak.

Question 2

When Snow looked at his completed map, what pattern did he observe in the data and what hypothesis did the data suggest to him?


He observed that most of the affected households took their water from the same pump and he formed the hypothesis that something in the water had caused them to develop cholera.

Question 3

Snow’s experiment was to remove the handle from the pump, so the local population had to get their water from elsewhere. Why is it crucial to the scientific method that Snow continued to measure the number and location of cholera deaths after the pump handle was removed?


A scientific hypothesis must be testable in order to rule it in or out as a valid explanation. Snow’s hypothesis that water from the Broad Street pump was the source of cholera would have remained just a ‘guess’ unless he put it to the test by cutting off this water supply and measuring the subsequent decline in the number of cholera deaths.

Snow’s experiment demonstrates the two essential components of what scientists call a causal association (sometimes referred to as a causal ‘correlation’), i.e. he found evidence that a specific event is the cause of a specific outcome. In Snow’s case, the two components of his evidence can be expressed simply as:

  • intact pump is associated with many cases of cholera, and
  • removing the pump is associated with declining cases of cholera.

We can illustrate the problems in proving a causal association and some other aspects of the scientific method with an even earlier experiment to prevent an infectious disease, this time involving smallpox.


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