John Snow And The Cholera Story – Free PDF Download



John Snow

  • John Snow was an English physician and a leader in the development of anaesthesia and medical hygiene.
  • He is considered to be the Father of modern epidemiology.
  • His work in tracing the source of a cholera outbreak in Soho, London, in 1854 is considered be one of the most important developments in epidemiology.
  • He spent several decades studying cholera in a systematic way.
  • Snow’s findings inspired fundamental changes in the water and waste systemsof London, which led to similar changes in other cities, and a significant improvement in general public health around the world.



  • Cholera is an infectious disease that became a major threat to health during the 1800s.
  • There were large epidemics of cholera in Europe and America that killed thousands of people.
  • Even today, cholera affects an estimated 3–5 million people worldwide and causes 28,800–130,000 deaths a year.
  • Cholera is an infection of the small intestine by the bacterium Vibrio cholerae
  • The classic symptom is large amounts of watery diarrhoeathat lasts a few days. Vomiting and muscle cramps may also occur.
  • The loss of fluids may result in sunken eyes, cold skin, decreased skin elasticity, and wrinkling of the hands and feet
  • Diarrhoea can be so severe that it can lead to death within hours.
  • Severe cholera, without treatment, kills about half of affected individuals.


  • The miasma theory held diseases—such as cholera—were caused by a miasma, a noxious form of “bad air’’  emanating from rotting organic matter.
  • The theory was eventually given up by scientists and physicians after 1880, replaced by the germ theory of disease: specific germs, not miasma, caused specific diseases

London in 1800’s

  • The sanitary conditions in London in the 1800s were poor, to put it kindly.
  • Most* people didn’t have running water or modern toilets in their homes.
  • They used town wells and communal pumps to get the water they used for drinking, cooking and washing.
  • Septic systems were primitive and most homes and businesses dumped untreated sewage and animal waste directly into the Thames River or into open pits called “cesspools”.

Cesspit or Cesspool

  • Some of London’s water came from shallow wells within the city and was delivered by the bucketful.
  • More prosperous areas had piped water provided by private water companies from the Thames itself.
  • The invention of the flush toilet or water closet at the end of the eighteenth century made things worse.
  • Well-off households that could afford a proper water supply could now dispense with a smelly cesspool and simply flush all their waste away – and out into the Thames

The Water companies

  • Water companies often bottled water from the Thames and delivered it to pubs, breweries and other businesses.
  • Later they started piping this water to homes.

Cholera comes Knocking

  • 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, leading to 14000 deaths.
  • Snow had long believed that water source was the cause of cholera.
  • He showed that the Southwark and Vauxhall Waterworks Company was taking water from sewage-polluted sections of the Thames and delivering the water to homes, leading to an increased incidence of cholera.
  • 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 Outbreak of 1854

  • In late August of 1854, cholera broke out in the Soho area of London.
  • 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 went to homes and learned from relatives that the vast majority of them had obtained their water from the Broad St. pump.



The exceptions –  Workhouse and Brewery

  • There was one significant anomaly—none of the workers in the nearby Broad Street brewery contracted cholera.
  • As they were given a daily allowance of beer, they did not consume water from the nearby well
  • Snow also noted that there was an extremely low incidence of cholera at a nearby work house.
  • They had their own water supply.

Other Exceptions

  • 3 deceased were children who went to school near the pump in Broad Street.
  • Few workers who walked to their jobs and used the pump on the way.
  • A mother and daughter living 3 km away but had fetched water from the Broad street pump few days back.
  • Snow was able to track down 197 victims, the vast majority of whom lived within walking distance of the pump.
  • Snow 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 authorities were not convinced, but agreed to remove the pump handle as a precaution. The epidemic quickly subsided.

The Index patient

  • Some months later an associate of Snow’s stumbled upon the records of an infant who had died of diarrhoea 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 diarrhoea 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.

John Snow’s Contribution 

  • He proposed a new hypothesis for how cholera was transmitted.
  • He tested this hypothesis systematically by making comparisons between groups of people.
  • He provided evidence for an association between drinking from the Broad St. well and getting cholera.
  • He argued for an intervention which prevented additional cases (removal of the pump handle).
  • 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.


Did John Snow get accolades for his discovery?

  • NO
  • Despite the success of Snow’s theory in stemming the cholera epidemic in Soho, public officials still thought his hypothesis was nonsense.
  • They refused to do anything to clean up the cesspools and sewers.
  • The Board of Health issued a report that said, “we see no reason to adopt this belief” and shrugged off Snow’s evidence as mere “suggestions.

John Snow did know a thing or two, after all

  • In 1883 a German physician, Robert Koch, took the search for the cause of cholera a step further when he isolated the bacterium Vibrio cholerae, the “poison” Snow contended caused cholera.
  • Koch determined that cholera is not contagious from person to person, but is spread only through unsanitary water or food supply sources, a major victory for Snow’s theory.
  • The cholera epidemics in Europe and the United States in the 19thcentury ended after cities finally improved water supply sanitation.




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