HLTH 485
Case Study Assignment :
Case Study I: Snow on Cholera
HLTH 485 - Principles of Epidemiology and Biostatistics for Business
Sara Smith
October 10, 2025
A: Observations on cholera
1. Concerning John Barnes, how was cholera communicated? What were the modes of
disease transmission? What is the correct epidemiological term for the modes of
transmission that were identified?
Concerning John Barnes, cholera was communicated through contaminated water that
had been exposed to sewage. The main mode of transmission was fecal-oral, meaning
people became infected by ingesting water or food contaminated with feces from an
infected person. The correct epidemiological term for this mode is indirect transmission
via a vehicle-borne route.
2. How long is a fortnight?
A fortnight means a period of two weeks or fourteen days.
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3. In the examples or circumstances presented, what were the various modes of
transmission stated or at least alluded to? Which were correct? Which were incorrect
and why? What role did personal hygiene and sanitation (including food preparation and
hand washing) play in the transmission of the disease and continuation of an outbreak?
In the examples described, the modes of transmission included contaminated water,
contaminated food, poor sanitation, and person-to-person contact. The correct mode
was the waterborne transmission through contaminated drinking water. The incorrect
modes included beliefs that cholera spread by bad air or through casual touch, which we
now know are false. Poor hygiene, inadequate handwashing, and unsanitary food
preparation allowed contaminated waste to enter water sources, worsening the
outbreak.
4. Several instances of causal association were presented or alluded to in the examples
presented in the case. List the various instances of association you can identify from the
examples or situations presented in the case. What role did social class, poverty, and
housing arrangements play in association? What role did water play in association?
The case study showed several causal associations for example, between contaminated
water sources and higher cholera rates, between crowded, impoverished living
conditions and increased disease, and between better sanitation and lower infection
rates. Social class and poverty played a role because poorer areas had less access to
clean water and proper waste disposal. Housing arrangements such as shared wells also
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increased exposure. Water was the central element of association, as it carried the
bacteria from fecal matter into households.
5. Describe the disease cholera as presented in the Snow case. Describe cholera as it is
known today.
In the Snow case, cholera was described as a sudden, severe disease causing watery
diarrhea, vomiting, dehydration, and rapid death if untreated. At that time, its cause was
unknown. Today, cholera is known to be caused by the bacterium Vibrio cholerae, which
releases a toxin that leads to massive fluid loss through the intestines. Modern
treatment involves rehydration therapy and antibiotics, and prevention focuses on clean
water and sanitation.
6. What hypotheses were developed by John Snow about the cause (etiology), signs and
symptoms, spread and course of the cholera disease? How do the observations and
hypotheses of Snow conform with modern understanding and knowledge of cholera?
John Snow hypothesized that cholera was spread by contaminated water rather than by
foul air. He observed clusters of cases around specific water pumps, particularly Broad
Street, and concluded that the disease followed water supply lines. His ideas align with
modern knowledge. Snow essentially identified waterborne transmission decades
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before bacteria were discovered. His hypotheses about cause, spread, and prevention
were remarkably accurate for his time.
7. What is different about those who develop cholera compared with those who do not?
Those who developed cholera differed from those who did not based on their exposure
to contaminated water and their immunity or resistance. People who avoided certain
wells, drank beer or boiled water, or lived in areas with cleaner supplies were less likely
to get sick. Some individuals may also have had stronger immune systems or fewer
bacteria ingested, explaining variations in illness.
8. What epidemiological phenomenon can be observed in the Locksbrook, near Bath,
example?
In the Locksbrook, near Bath example, Snow observed that people who used water from
a contaminated river developed cholera, while those who used clean spring water
remained healthy. This illustrated an epidemiologic association between exposure to
contaminated water and disease occurrence, a clear example of how environment and
exposure influence disease distribution.
B: Cholera and the Broad Street Outbreak
1. What are the time factors and implications for this case? Explain the time lag from
attacks in Figure I.2 to Deaths in Figure I.3.
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The main time factor in this case was the short interval between exposure and illness.
The epidemic rose and fell rapidly, lasting only about two weeks. The lag between
attacks in Figure I.2 and deaths in Figure I.3 shows the natural course of cholera, people
first became ill, then dehydration and shock caused death one or two days later. This
demonstrates how the timing of exposure, onset, and death helped Snow understand
the speed and severity of the outbreak
2. What are the place factors and implications for this case? Compare the workhouse to
the brewery to the pub. Discuss migration and its effect on the epidemic. What place
issues are important to this case?
Place factors were crucial because the clustering of cases around the Broad
Street pump showed the disease’s geographic pattern. In comparison:
❖The workhouse had few cases because it had its own private well.
❖The brewery workers stayed healthy because they mostly drank beer instead of
pump water.
❖The nearby pubs and homes that used the Broad Street pump were heavily
affected.
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Migration also influenced the epidemic people who left the area early avoided exposure, while
those arriving after contamination risked infection. These place patterns confirmed that
contaminated water was driving the epidemic.
3. What did the well persons do differently than the ill—those who got the disease (for
example the inmates in the workhouse)?
The people who remained well avoided the contaminated water. For example, brewery
workers drank beer, and workhouse inmates used a different water source. The ill
individuals, in contrast, consumed water from the Broad Street pump. This behavioral
difference, drinking or not drinking that water, was the main determinant of illness.
4. From Figure I.2, what is the index case? What date is the beginning of the epidemic?
What other time factors are discerned from this chart? How did Snow establish the time
of onset?
From Figure I.2, the index case (the first known infection) occurred in late August 1854,
marking the beginning of the epidemic. The number of cases rose sharply in early
September. Snow determined the onset time by mapping household deaths and
interviewing families about when symptoms first appeared. His use of precise timing and
mapping helped reveal the short incubation period and explosive nature of the
outbreak.
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5. What accurate observations were made about wells, cesspools, and ground water, the
purity of the water, and its contamination? What inaccurate and misunderstood
observations were made about the water, its flow, and its contamination?
Snow and others made several important accurate observations: contaminated wells
could transmit disease, cesspools leaked into groundwater, and water near sewers was
unsafe. However, some early investigators incorrectly believed water “purified itself” as
it flowed underground or that odor (miasma) caused illness. These misunderstandings
delayed acceptance of Snow’s theory that fecal contamination not smell spread cholera.
6. What evidence did Snow use to establish the fact that a cholera epidemic was
occurring? Did Snow clearly demonstrate the cause and source of the outbreak of
cholera in the Golden Square area? Explain.
Snow gathered epidemiological evidence through mapping cases, interviewing residents,
and comparing affected and unaffected groups. The clustering of deaths near the Broad
Street pump and the absence of cases among people who avoided it confirmed that an
epidemic was occurring. Yes, Snow clearly demonstrated that the source of the outbreak
was the Broad Street pump, establishing the connection between contaminated water
and cholera transmission.
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7. What were Snow’s initial and basic hypotheses concerning the epidemic? What
processes and procedures did Snow use to establish his hypotheses (prove or disprove
them) about the outbreak?
Snow’s initial hypotheses were that cholera spread through contaminated drinking water
and that removing access to that source would stop the epidemic. To test this, he
conducted a natural experiment by comparing groups who used different water
supplies. His systematic approach collecting data, analyzing patterns, and ruling out
alternative explanations was an early example of analytic epidemiology. His results
supported his hypothesis and contradicted the miasma theory.
8. The obvious control measure applied was the removal of the handle from the Broad
Street pump. What role did the removal of the pump handle play in the decline of the
epidemic? Did the removal of the pump handle have an effect on the control epidemic?
What other social and political roles did removing the handle from the pump play?
Removing the Broad Street pump handle symbolized Snow’s conclusion and served as a
control measure. Although the epidemic was already declining, its removal prevented
further use of contaminated water and reassured the community. Socially and politically,
the act drew attention to the importance of sanitation reform and public health
interventions. Even if it had limited direct impact on the curve, it became a powerful
step in disease prevention and public health history.
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C: Cholera Epidemic of 1853 and Two London Water Companies
1. What are the various aspects of the person presented in the above water company case
concerning the epidemic of cholera, especially as the person concept relates to the
households which were recipients of the supplies of the water as compared to those
who used different water supplies?
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In this case, the concept of person focused on individuals and households who received
water from two different companies, Southwark and Vauxhall versus Lambeth. People
supplied by the Southwark and Vauxhall Company experienced much higher rates of
cholera because their water was drawn from polluted parts of the Thames River. In
contrast, households supplied by the Lambeth Company had far fewer cases, since their
intake was moved upstream to cleaner water. This comparison showed that exposure to
contaminated water determined who became sick, emphasizing the person-level
differences in risk based on water source.
2. Concerning the water supply system and structure from the two different water
companies, explain the soundness of the research approaches. Is this a descriptive or
analytic epidemiologic research design?
Snow’s approach in comparing the two companies was methodologically sound for his
time. By studying two populations that were similar in all respects except for their water
supply, he created what we now call a natural experiment. This design allowed him to
observe outcomes without manipulating variables directly. His research design was an
analytic epidemiologic study because he compared two exposure groups to identify a
causal relationship between contaminated water and cholera cases.
3. What were some of the practical problems and barriers that Snow faced which slowed
his inquiry of the cholera epidemic of 1854? How would this relate to modern day
epidemiological investigations?
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Snow faced many practical barriers during his investigation. He lacked modern
laboratory tools to identify bacteria, and official records were often incomplete or
inaccurate. Communication and transportation delays also slowed his data collection.
Additionally, many city officials and medical professionals still believed in the miasma
theory, making it difficult for Snow to gain acceptance for his waterborne hypothesis.
These challenges are similar to modern-day epidemiology issues such as misinformation,
lack of data access, and limited resources during emerging outbreaks.
4. “As the epidemic advanced, the disproportion between the number of cases in houses
supplied by the Southwark and Vauxhall Company and those supplied by the Lambeth
Company, became not quite so great. . . .” From an epidemiological point of view why
was this so? Give detailed epidemiological reasoning and discussion addressing person,
place, and time issues.
As the epidemic advanced, the difference between cholera cases in homes supplied by
Southwark and Vauxhall versus those supplied by Lambeth became smaller.
Epidemiologically, this occurred because the time, place, and person factors changed.
Over time, contamination spread further along the Thames, and migration of people
between areas caused mixing of exposures. Some Lambeth households might have
consumed other water sources or food prepared with contaminated water. The initial
clear difference diminished as exposure patterns overlapped, demonstrating how
environmental and population changes affect disease distribution.
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D: Epidemiologic Issues
1. What did Snow observe regarding the association between city size and the length of
the epidemic?
Snow observed that the larger the city, the longer the cholera epidemic lasted. In big
cities like London, population density, poor sanitation, and shared water sources made it
easier for the disease to spread and harder to control. Smaller towns, with fewer shared
water sources and less contamination, saw shorter epidemics. This observation helped
Snow link urban living conditions to disease persistence.
2. Snow suggested several reasons for the failure of contaminated water to produce
disease in all who consume it. Why is it that not everyone became sick or died when
they consumed the cholera-causing bacterium Vibrio cholera?
Snow suggested several reasons why not everyone who drank contaminated water
became ill. The amount of bacteria a person ingested varied, and some individuals might
have had partial immunity or stronger health resistance. Others boiled their water or
drank beverages like tea or beer, which killed the bacteria. Additionally, the bacteria
might not have survived long in cooler or cleaner containers. These factors explain why
exposure did not always lead to disease, even in the same household.
3. Which of Snow’s observations about time and seasons and their effects on epidemics
were correct and which were incorrect? Why?
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Snow correctly observed that season and time influenced cholera epidemics. He noticed
that outbreaks were more common in warmer months, which is true because higher
temperatures support bacterial growth and contamination of water. However, he was
limited by the lack of microbiological knowledge, so some of his seasonal assumptions
were incomplete—he couldn’t fully explain the bacterial life cycle or environmental
conditions affecting it. Still, his conclusions about time and weather influencing
epidemics were largely accurate.
4. Several different theories and hypotheses have been presented. Which of these are
consistent with known scientific and biomedical knowledge and common sense? Which
of these are not? Why?
Among the various theories of disease spread at the time, Snow’s waterborne
hypothesis was consistent with modern science and common sense. His idea that fecal
contamination of drinking water caused cholera is now known to be correct. The
miasma theory, which blamed bad air or odors, was inconsistent with scientific evidence
and failed to explain why cholera clustered around specific wells. Other beliefs, like
divine punishment or atmospheric changes, lacked biological basis. Snow’s evidence-
based reasoning became the foundation of modern epidemiology.
References
Merrill, R. M. (2022). Case study I: Snow on cholera. In Introduction to epidemiology
(9th ed., Appendix I). Jones & Bartlett Learning.
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