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Snow on Cholera

Read this topic in the printed workbook (or online) and then work through the interactions

Cholera reappeared in London in June 1853. John Snow asked permission to obtain (from William Farr) the addresses of people who died of cholera in the districts which received water supplies from both the Southwark and Vauxhall and the Lambeth companies.

He then visited the homes of all recorded cholera deaths in these districts, to get information about which company supplied water to the household. The table shows his results for the first 334 deaths.

Source of water No. of Deaths
Southwark and Vauxhall company 286
Lambeth company 14
Direct from the river 22
Pumpwells 4
Ditches 4
Unknown 4


We might just note that John Snow did a very thorough job - he managed to get information from 330 out of the 334 households. He did this by going from house to house to make enquiries. Today, this type of investigation is often called "shoe-leather epidemiology", meaning that it involves so much walking that your shoes may wear out!

In order to estimate the denominator, Snow obtained information on the number of houses in London whose water was supplied by each of the two water companies.

Snow noted the source of water in the houses of all those who died of cholera from 8th July to 26th August 1854, as shown in this table:

Source of water Total no. houses supplied No. cholera deaths
Southwark and Vauxhall 40,046 1263
Lambeth 26,107 98
Other 256,423 1422

Take a few minutes to think about whether there are problems with using the number of houses supplied by each company as a denominator, then click below for our interpretation.

Using the number of houses as a denominator

However for this example we will assume there was no difference in the number of people in each house in the areas supplied by the two water companies.


So John Snow started with descriptive epidemiology, obtaining information on the numerator (the number of cholera deaths) and the denominator (the number of people supplied with water) so that he could describe the number of cases of cholera in different areas in relation to the size of the population at risk.

He then went on to compare the death rates from cholera in the two areas. He used this information to calculate how much more risky Southwark and Vauxhall water was compared to Lambeth water. In other words, he looked for an association between water source and the risk of death from cholera.

We call this analytical epidemiology meaning studies in which we attempt to explain differences in patterns of disease by examining associations, and identifying possible causes of the disease.

We shall return to John Snow's investigations later.

Aims and Objectives
What is Veterinary Epidemiology?
Snow on Cholera
Exposures & Outcomes
Observe or Intervene
Role of Epidemiology
Types of Investigation
A Clinical Problem
Population Perspective