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At that time Snow realised that quite a number of miners contracted the disease in the absence

of swamps and sewers. He realised that the first symptoms of the disease involved the

intestinal tract. He concluded that, if poison from polluted air spreads the disease, then the

respiratory tract should be involved. It made more sense to think of food or water being the

vehicle for the disease, and he started to consider the germ theory as an alternative way in

which the cholera was spreading.

Cholera in London and Snow’s investigation of three water companies

The resurfacing of cholera in London provided the opportunity to test the hypothesis

that polluted water is linked to the deadly disease. To fully apprehend Snow’s gift for

conducting epidemiological surveys by recognising the unique features of a situation which

best served the investigation, one must know how water was distributed in London at that

time. Besides street pumps, which provided water free of charge to the population, companies

also delivered piped water to houses, a service for which people had to pay. Snow

concentrated on three companies, namely the Southwark and Vauxhall Company, which were

drawing water from the Thames along an area where sewage outlets polluted the water and

the Lambeth Water Company which, in contrast, drew water from an area more upstream and

above from the sewage outlets. Snow started before August with a comparatively small

survey in two subdistricts of South London, namely Lambeth and Kennington, and found that

the majority of people dying of cholera lived in houses receiving water from the Southwark

and Vauxhall Company. Together with Dr. Whiting, he expanded his survey over a period

from 8

th

of July to the 5

th

of August. To ask for help was necessary because they had to visit

each house with a cholera death in order to find out which company delivered water to that

particular dwelling. It was found that, out of 44 deaths, 38 victims lived in houses receiving

water from the companies piping in polluted water, and only 14 deaths were recorded from

houses having water from the Lambeth Company. Finally Snow resorted to a large statistical

investigation the results of which are given in Table1.

TABLE 1. Death rate from cholera by source of water. Rate per 10,000 houses. London

epidemic of 1853–54. Snow’s Table IX.

No. of Houses Cholera Deaths Rate per 10,000

Southwark and Vauxhall

40.046

1.263

315

Lambeth

26.017

98

37

Rest of London

256.423

1.422

59

The denominator data, that is the number of houses served by each water company,

were available from parliamentary records. For the numerator data, however,

a house-to-

house canvass was needed to determine the source of the water supply at the address of each

cholera fatality (the “bills of mortality” showed the address, but not the water source). The