HomeWork - Case Study - John Snow and Epidemiology

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John Snow and the Origin of Epidemiology
Part I—Beginnings
John Snow was born in York, England, in 1813, the first of 9

children of a working-class family. Snow’s wealthy and well-
connected uncle, arranged an apprenticeship for his nephew
with a surgeon-apothecary, one of the two types of health
care providers in 19th century London. Physicians were
graduates of the medical programs at Oxford or Cambridge
while surgeon-apothecaries went through a longer
apprenticeship, attending classes part-time at smaller medical
schools. John Snow moved to Newcastle at the age of 14 to
apprentice with William Hardcastle. It was in Newcastle, near
the end of Snow’s apprenticeship, that he first encountered
cholera as it arrived in England in 1831.
In his medical studies, Snow learned the prevailing humoral

model of disease, which held that health depended on the
balance of four humors: blood, phlegm, black bile, and yellow
bile. Diseases resulted from an excess or deficit in one of
these four humors. To correct the problem, physicians would
use leeches to bleed patients or purgatives to cause diarrhea
or vomiting. The humoral model eventually was replaced
with the miasma model of disease, which suggested that
diseases were caused by pollution or “bad air.” At the time,
the Germ Theory of Disease had not been established, and physicians didn’t fully understand the nature of disease
and its transmission.
After being released from his apprenticeship, John Snow was one of the first physicians to study and calculate
dosages for ether and chloroform as surgical anesthetics. It was his work with anesthesia and gases that made him
doubt the miasma model of disease.
1. Compare the two different types of medical professions of the 19th century. What modern professions would
compare to these?
2. *Search cholera and list the major symptoms of the disease. Why is it sometimes called “The Blue Death?”
3. Compare the humoral model of disease (part 1) to the miasma model of disease. Which do you think is closest to
our modern understanding of disease?
Part II: Sanitation in the 19th Century
London in the middle of the 19th century contained 2.5 million

people, housed in 30 square miles, a population density greater
than present-day Manhattan. The Soho district of London had
a serious problem with filth due to the large influx of people and
a lack of proper sanitary services: the London sewer system had
not reached Soho. Many cellars had cesspools underneath their
floorboards.
A cesspit (cesspool) was an underground holding tank used for

the storage of feces. Some pits were emptied when they became
full; cleaned out by tradesmen using shovels and horse-drawn
wagons. Some cesspits were designed to allow liquid to leech
into the soil. Because of the population density in London,
many of these cesspits were overflowing; waste accumulated in
basements, courtyards, and even the streets. Since the
cesspools were overrunning, the London government decided to
dump the waste into the River Thames.
Because of the problems of waste disposal, few Londoners had

a source of drinking water uncontaminated by human sewage. At
that time, a total of nine different water companies supplied
Londoners with water, obtained from either shallow wells or the
Thames River. Some companies had their intake pipes farther
upstream than others. Water obtained from pipes downstream were more likely to be contaminated with human waste.
4. What was it like to live in London in the early 19th century?
5. Examine the diagram showing a cesspit. Before houses installed cesspits, chamber pots were dumped into the
streets. What were some advantages to having a cesspit? What were the disadvantages?
6. How does changing the location of the pipes (either upstream or downstream) improve water quality? Sketch an
image of the river and the intake pipes and sewer pipes to show the ideal location.
Part III: Outbreak
On 31 August 1854, after several other outbreaks had occurred elsewhere in the city, a major outbreak of cholera
struck Soho. By September, 500 people had died and the mortality rate was 12.8 percent in some parts of the city.
By the end of the outbreak, 616 people had died. John Snow later called it "the most terrible outbreak of cholera which
ever occurred in this kingdom."
The germ theory was not created at this point (as Louis Pasteur would not create it until 1861), so Snow was unaware
of the mechanism by which the disease was transmitted, but evidence led him to believe that it was not due to
breathing foul air as the miasma model would suggest. He first published his theory in an essay On the Mode of
Communication of Cholera in 1849 which proposed that cholera was transmitted in water. The essay received
negative reviews in the Lancet and the London Medical Gazette. However, a reviewer made a helpful suggestion in
terms of what evidence would be compelling: the crucial natural experiment would be to find people living side by side
with lifestyles similar in all respects except for the water source.
Snow sought ways of strengthening his argument by carrying out the crucial experiment sought by the Medical
Gazette’s reviewer. He went door to door interviewing families of cholera victims. Snow began marking cholera
deaths on city maps, and patterns began to emerge. He mapped out the locations of individual water pumps and
generated cells which represented all the points on his map which were closest to each pump.
7. Based on the data shown in the map, which pump is the most likely source of the cholera infection? Circle all
pumps that might also be suspect in this investigation.
8. The cases near Saville Row might be considered an outlier as they have their own pump nearby. What questions
would Snow want to ask family members in this area?
Part IV: Snow Makes His Case
In Snow's own words:
On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the [Broad
Street] pump. There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of
these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street,
as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were
children who went to school near the pump in Broad Street...
The result of the inquiry, then, is, that there has been no particular outbreak or prevalence of cholera in this part of
London except among the persons who were in the habit of drinking the water of the above-mentioned pump well.
I had an interview with the Board of Guardians of St James's parish, on the evening of the 7th inst [September 7],
and represented the above circumstances to them. In consequence of what I said, the handle of the pump was
removed on the following day.
~John Snow, letter to the editor of the Medical Times and Gazette
Although Snow's chemical and microscopic examination of a sample of the Broad Street pump water was not able to
conclusively prove its danger, his studies of the pattern of the disease were convincing enough to persuade the St
James parish authorities to disable the well pump by removing its handle. At this point, John Snow had partnered with
Reverend Henry Whitehead who assisted with interviewing families and tracking the disease. Whitehead succeeded in
identifying an earlier case, an infant living in a house a few feet from the Broad Street pump who died from diarrhea
two days before the cholera outbreak was officially recognized.
After excavation of the Broad Street well, it was found that it had been dug only three feet from an old cesspit that had
begun to leak fecal bacteria. A mother of the baby who had contracted cholera had its diapers washed into this cesspit
and was likely the source of the original infection.
9. What was John Snow’s original hypothesis and how did it conflict with prevailing models of health and disease?
10. Why would evidence of cholera in people living side by side, differing only in water supply, provide critical
evidence?
14. Snow found that none of the monks in the adjacent monastery contracted cholera. They drank only beer, which
they brewed themselves. Does this mean that beer made with contaminated water is safe to drink? How could you
test this?
Part V: The Aftermath
Unfortunately, after the cholera epidemic had subsided, government officials

replaced the Broad Street Pump Handle. They had responded only to the urgent
threat posed to the population, and afterward they rejected Snow's theory. To accept
his proposal would have meant indirectly accepting the oral-fecal method
transmission of disease, which was too unpleasant for most of the public to
contemplate.
Although many continued to reject Snow’s explanation, some began to give it

grudging acceptance, often without acknowledging his contribution. Snow’s
vindication came at a meeting of the Medical Society where a member stood up
after such a presentation insisting that Snow be given credit. The pump is now a
historic site in London and is located in front of the John Snow Pub.
15. The basic questions of epidemiology focus on time and place: “why here” and
“why now”. What are the answers to these questions for the Broad Street outbreak?
16. Why was the death of the baby a significant observation for this study?
17. Epidemiology relies on non-experimental tests of hypotheses. What was Snow’s hypothesis and how did he test it
without performing experiments.
18. Consider the term “non-experimental.” Given you had no ethical concerns with testing on humans, how would
you test the hypothesis in an “experimental” way? (Attach paper if necessary)

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Name(s) ________________________________________________

John Snow and the Origin of Epidemiology

John Snow was born in York, England, in 1813, the first of 9

In his medical studies, Snow learned the prevailing humoral

Part II: Sanitation in the 19th Century

London in the middle of the 19th century contained 2.5 million

A cesspit (cesspool) was an underground holding tank used for

Because of the problems of waste disposal, few Londoners had

4. What was it like to live in London in the early 19th century?

Part IV: Snow Makes His Case

In Snow's own words:

Unfortunately, after the cholera epidemic had subsided, government officials

Although many continued to reject Snow’s explanation, some began to give it

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