John Snow - The Father of Epidemiology

Cholera is an infectious disease that became a major threat to health during the 1800s. The story has been elegantly told in The Ghost Map by Steven Johnson, who describes the conditions in London in the 1800s situation in the brief video below.

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Cholera continues to be a problem throughout the world today (see Cholera in Haiti). The next video describes the cholera epidemic in Haiti in 2010.

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Link to more on cholera in general

Link to more on The Ghost Map by Steven Johnson

Link to more on cholera in Haiti

In the 1800s there were large epidemics of cholera in Europe and America that killed thousands of people. John Snow (shown below) was a physician in London who spent several decades studying cholera in a systematic way. He is most often credited with solving an outbreak of cholera that occurred in London in 1854 (the outbreak is described below), but his studies of cholera were much more extensive than that. The first cholera epidemic in London struck in 1831, when Snow was still an apprentice. Another large epidemic occurred in 1848 and lasted through 1849.

Link to more on John Snow

Dr. John Snow

The prevailing opinion was that cholera was spread either by miasmas or by person-to-person contact, Snow began examining the victims and found that their initial symptoms were always related to the gastrointestinal tract. Snow reasoned that, if cholera was spread by bad air, it should cause pulmonary symptoms, but since the symptoms were gastrointestinal, perhaps it was transmitted by water or food consumption. In fact, cholera is caused by the bacterium, Vibrio cholera, which is transmitted by the fecal-oral route, that is by ingestion of water or food that is contaminated with sewage.

In August 1849 Snow published a paper entitled "On the Mode of Communication of Cholera" in which he presented his theory that the disease was acquired by ingestion of contaminated water, but his theory did not get much traction with the medical establishment. The epidemic ended in 1849, but Snow continued to collect data on the pattern of of disease and began finding evidence that linked cholera to specific sources of water.

Many Londoners received their water from hand pump wells (below) that were located throughout the city.

However, increasing numbers of businesses and homes had water piped from the Thames River by private companies. Snow learned from municipal records that two private companies supplied piped in water to the areas that were primarily affected by cholera. Some consumers were supplied by the Lambeth Company, while others were supplied by Southwark & Vauxhall. The map of London below shows the areas of London supplied by these two water companies.

Map of London showing the areas receiving water from the two major water companies.

Southwark & Vauxhall pumped water from a more downstream location that was clearly contaminated, and the rates of cholera were clearly higher in their customers compared to those supplied by the Lambeth Company. Nevertheless, many were unconvinced by his findings, since Snow had not actually demonstrated that the water contained something that could cause cholera.

Water Company

# Houses Served

# Cholera Deaths

Death Rate per 10,000 Houses

Southwark & Vauxhall








The Rest of London





 In late August of 1853, cholera broke out in the Broad Street area of London, and the residents panicked; many began to flee. A hand pump was located right on Broad Street, and Snow was immediately suspicious. Water samples looked fine, but Snow persisted and began to collect detailed information on where the victims had gotten their drinking water. He obtained the names and the addresses of the first 83 victims who had died by the end of the first week. He went to their homes and learned from relatives that the vast majority of them had obtained their water from the Broad St. pump.

On Sept. 6 Snow appeared at the meeting of the local Board of Guardians and presented his evidence that the pump was the source of the outbreak. He argued that the pump handle should be removed in order to prevent further contamination. The board was not convinced, but agreed to remove the pump handle as a precaution. The epidemic quickly subsided.

The investigation continued. Ultimately, Snow was able to track down 197 victims, the vast majority of whom lived within walking distance of the pump. It was also noted that there was an extremely low incidence of cholera at a nearby work house and also at the Lion Brewery, and both of these businesses had their own water supply. The workers at another large business used water from the Broad St. pump, and their workers had a substantial incidence of cholera.

The map below shows the location of the pump, and the home or business location of the victims is shown by stacks of small dark marks that are clearly clustered around the pump. This type of map, which marks the location of disease cases, is now referred to as a "spot map."

A spot map of the Broad Street area of London in 1852. A series of black disk=shaped marks indicate the location of cholera victims, and they appear to be clustered around the Broad Street water pump.

Initial examination of the well failed to show any problems, casting doubt on Snow's conclusions, and the pump was reopened without incident. However, some months later an associate of Snow's stumbled upon the records of an infant who had died of diarrhea at the very beginning of the outbreak. The timing of her death indicated that she had been the first cholera case. Upon questioning, the mother said that she had emptied a pail of the infant's diarrhea into a cesspool in front of their house immediately adjacent to the water pump. The cesspool and the pump well were than excavated, revealing that the cesspool, which was within three feet of the well, was leaking, and the wall of the well was decayed, allowing the contamination from the cesspool to seep in. In retrospect, it appeared that once the child died, there was no further contamination of the well, and the epidemic ended.

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The graph below is an "epidemic curve" that plots the number of new cases that occur over time. This graph shows that the number of cholera deaths abruptly increased at on the last day of August, quickly rising to a peak of 130 cases in the first two days of September. After that, the number of daily deaths gradually fell back to near zero over the next three weeks.

If one knows the incubation period for the disease, the shape of an epidemic curve can provide clues regarding the source of the epidemic. Cholera has an incubation period of only 1-3 days, and this graph indicates that new cases occurred over a period of about 10 days. i.e., over multiple incubation periods. This suggests a "continuous source" epidemic, because new cases are not confined to a single focal exposure that would produce a spike in cases within a single incubation period.

In retrospect, Snow made several important contributions to the development of epidemiologic thinking: