“It marked a turning point in the battle between urban man and Vibrio Cholera, because for the first time a public institution had made an informed intervention into a Cholera outbreak based on a scientifically sound theory of disease.”[1]
The above quote from Steven Johnson’s The Ghost Map demonstrates a shift in scientific methodology that occurred during the mid-1800s in the western world. Although Johnson portrays John Snow, the man he credits with discovering Cholera as a waterborne disease, as a hero, valiantly fighting against the common misconceptions of miasma, what is really more remarkable is the methods Snow employed to reach this conclusion. The most significant impact of John Snow’s research came from his use of the new scientific methods of deduction based on evidence. Scientific deduction methods, like those used by Snow, continue to influence public health and modern scientific approaches to the spread of disease today. By analyzing statistics, tracking specific cases, and eventually mapping where a disease appeared, individual scientists and public health boards of the mid-1800s exercised a new approach to the scientific testing of vehicles of disease transmission.[2] The normalization of this new scientific research method is what stands out as John Snow’s legacy in the medical community today.
At the start of London’s1854 Choleraoutbreak, Snow began his investigation byanalyzing the statistics of Cholera deaths inLondon’s 1850s death tables. These tables were compiled and maintained by William Farr, London’s Statistical Superintendent of the General Registrar Office.[3] Farr also used his statistical research on causes of death to develop his own theory of elevation. Farr employed the newly popular method of statistical analysis to reach his conclusion that people living at a higher elevation were less likely to contract Cholera. Although Johnson points out how Farr’s conclusion turned out to be a “classic case of correlation being mistaken for causation,” his use of death statisticsshows how statistical analysis was becoming the norm for medical research.[4] This method also extended outside the scientific circles of research into the more mainstream public health officials’ work. Dr. Robert Barnes, the Medical Office of Health in Shoreditch, England during the 1850s and ‘60s, considered the “diligent search into the mortality records and statistical calculation [of medical illness to be the proper way to provide evidence] of the sanitary conditions of Shoreditch.”[5] Barnes’ use of statistics to analyze deaths in his community and their relation to sanitary conditions, coupled with the evidence provided by Snow and Farr, demonstrates how the method of statistical analysis was becoming more popular in scientific and public health fields of the 1850s.
While statistical analysis made up one component of the new scientific methods used in London during the 1854 Cholera outbreak, Snow and his contemporaries also investigated their communities by talking to patients and recoding the specific living conditions they witnessed. In fact, Dr. Barnes of Shoreditch believed it must be his primary responsibility “to investigate and determine the conditions under which the vast population … of Shoreditch is living … [in order] to make an elaborate and minute medical survey of the parish.”[6] Johnson explains how this spirit of investigation also influenced Snow, as he believed “whatever numbers William Farr provided him [Snow] would have to be supplemented with local investigation.”[7] This type of local investigation proved pivotal in tracing the source of the outbreak to baby Lewis and the cesspool in front of 40 Broad Street.[8] The use of physical investigations like those done by Snow and Barnes became a staple in the public health duties of MOHs. As the collecting of knowledge on patients with specific diseases became even more important, MOHs in London and elsewhere began to use their personal investigations to identify disease patterns and report them to larger pubic health authorities.[9] Later public health inspectors like John Netten Radcliffe, George Buchanan and Richard Thorne-Thorne used the same type of personal investigations to show how Typhoid could also be transmitted through contaminated water.[10] Without the popularization of street-by-street investigations of public health officials, the use of statistics and detailed information on the spread of disease might not have become a mainstay in public health methods.
The methods of direct investigation and the use of statistics in disease analysis came together it one other scientific method pioneered by John Snow, the spot map. Snow combined his personal investigations of the Soho area with broad statistics collected by William Farr to develop aspot map of the Cholera outbreak. The high concentration of black bars around the Broad Street Pump provided a clear image of the spread of Cholera and its concentration around the pump.[11] Beyond giving a visual clarification to Snow’s waterborne theory, Johnson explains how Snow’s map “wedded state-of-the-art information design to a scientifically valid theory of Cholera’s transmission.”[12] While Dr. Barnes report shows how previous parish survey maps were used to identify dense areas of inhabitants, Snow’s map visually brings forward the scientific methodological advancements to provide what Johnson refers to as a “shorthand explanation of the science behind the theory.”[13]Later on, Medical Officer’s of Health like Edward Ballard of Islington, specifically used mapping in their investigations into other infectious diseases, like Typhoid. Much as Snow had done, Ballard mapped specific cases of Typhoid to individual houses in his attempts to visualize the spread of typhoid.[14] Spot mapping was one of Snow’s most valuable contributions to public health because it allowed MOHs like Ballard to visually analyze the spread of infectious diseases like Cholera and Typhoid.
John Snow’s Choleraspot map provided key visual evidence for the scientific and medical communities of London, and helped them accept his theory that Cholerawas spread through contaminated water. Physicians, scientists and public health officials began to see the value in an investigative scientific method with results backed by statistics. The use of these techniques began to separate medical findings from patient medicine and irregular healers, and “professionals” capitalized on the distance created. The scientific practice of these men allowed them to “justify” their social and medical superiority and influenced the increasing responsibility and authority physicians held in society during the 1880s and beyond.[15]Steven Johnson would argue the eventual acceptance of Cholera as a waterborne disease defined Snow’s contribution to medicine, but evidence shows it is really his use of scientific methods in developing this theory that definedhis impact on modern medicine.
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[1] Steven Johnson, The Ghost Map: The Story of London’s Most Terrifying Epidemic – and How It Changed Science, Cities and the Modern World (New York: Riverhead Books, 2006), 163
[2] Jacob Steere-Williams, “The Perfect Food and the Filth Disease: Milk-borne Typhoid and Epidemiological Practice in Late Victorian Britain,” Journal of the History of Medicine and Allied Sciences 65, 4 (October, 2010), 518.
[3]Johnson, 73.
[4]Johnson, 101.
[5] Robert Barnes, Preliminary Report on the Sanitary Condition of Shoreditch (10 April 1856), 4 in the Wellcome Digital Library, accessed October 18, 2015,
[6]Barnes, 4.
[7]Johnson, 79.
[8]Johnson, 178.
[9]Bynum, 87-88.
[10]Steere-Williams, 517.
[11] Peter Vinten-Johansen, The John Snow Archive and Research Companion: Chapter 12 Images, Figure 12.6, in the MATRIX Center for Digital Humanities and Social Sciences, accessed October 22, 2015,
[12]Johnson, 194.
[13]Barnes, 4.Johnson, 198.
[14]Steere-Williams, 522-23.
[15] Alan M. Kraut, Physicians and the New Immigration During the Progressive Era,” in Major Problems in the History of American Medicine and Public Health, ed. John Harley Warner and Janet A. Tighe (Boston: Wadsworth, 2001), 267.