Was John Snow lucky?

John Snow, the father of modern epidemiology, is a well-known figure to the public health practitioners, epidemiologists and students. In mid-19th Century he discovered that Cholera was caused by contaminated water.


Shortly put, the story behind that discovery was – in 1854 there was a cholera outbreak in Broad Street area, London. Around 600 people died in the first week of September around few blocks of Broad street. John Snow visited from house to house and collected information about the cholera death and water pump company that supplied water in those houses. Then he tabulated the water pump companies and corresponding deaths. He then created a dot map to locate the cholera death and observed clusters of deaths around some of the water pumps supplying water into those areas. This findings made him to reach a conclusion that contaminated water was the reason of cholera. Later he removed the pump handle which put an end to that deadly epidemic.

This part of story we all know! However, many of us do not know that at the same time William Farr, the Register General at that time, also found some patterns in cholera death with the altitude of the houses. He explained this to the “Miasmatic theroy” of disease which believed that diseases were transmitted by “miasma” or bad air/cloud. This theory of disease transmission was quite popular at that period. Farr was able to show that  cholera deaths were highest in houses with low altitude from sea level and decreased in a systematic fashion as the altitude increased.

Original map by John Snow showing the clusters of cholera cases in the London epidemic of 1854


John Snow and William Farr were in major disagreement about the cause of Cholera! Nobody knows, at that time, what is the cause of cholera!  But how John Snow came up with this conclusion? Was this a lucky-guessing? Was it a fluke? Or John Snow was ingenious?

As epidemiology students, we had discussion in our classroom about John Snow’s discovery? In the Q & A session, I politely asked my professor this question “Was John Snow lucky?”

She also started laughing. But, then, she pointed out some important facts necessary behind every scientific breakthroughs:

1. Firstly the context in which the research is conducted, is crucial. John Snow was able to study the cholera deaths because he was in England. If he was staying somewhere in Asia or Africa, he won’t be able to get all the data and study the cause of cholera! Even today, the epidemiologists in developed countries are able to answer many important research questions because they can avail necessary data from health registers, hospital records, drug records and so on.

2. Secondly, one needs to apply robust methodology  and John Snow used the observational methods to compare the death rates (though unstandardized!)  between different groups to test the research hypothesis.

3. Finally, John Snow had sharp mind and eyes! He was an anesthesiologist by profession. He hypothesized about the causal relation between contaminated water and cholera from another incidence! Few months before, one of the water pump company changed their water uptake to upstream of river Thames and no. of cholera deaths in the supplied areas suddenly dropped down! This intrigued John Snow to think about  such causal association!

She (my professor) explained wonderfully what it takes for a breakthrough discovery! However, I still believe you need touch of luck even though you possess all the qualities!

In epidemiology, many times we use 95% confidence interval for  estimates. My understanding about any success is – you can achieve all much as 95% with hard working & talent; but other 5% is absolute luck! Being in the right place, meeting the right people, asking the right question— all these are dependent on luck.

As an epidemiologist one can conclude it as “uncertainty or random factor” !!!!!


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