I need this in one hours
Snow and the Origin of Epidemiology; “You Know Nothing, John Snow.”
Adapted from https://docs.google.com/document/d/1BYGcSlV4ufjg-7dcKtPUYMD3O90fsBd44V-
Instructions: Read the passages and answer the questions.
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 anaesthetics. 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 leach 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
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
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
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
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 cluster of 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
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
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
12. What was John Snow’s original hypothesis and how did it conflict with prevailing models of
health and disease?
13. 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
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?