Substantiating the impact of John Snow’s contributions using data deleted during the 1936 reprinting of his original essayOn the Mode of Communication of Cholera

John Snow is considered a founder of modern epidemiology and his contributions to the field are highlighted in many introductory courses in medicine. Whereas all epidemiologists are familiar with the account of the Broad Street pump, fewer are familiar with the much larger and more compelling Grand Experiment that Snow exploited in South London. In his well-known essay On the Mode of Communication of Cholera, Snow devoted 25 pages to listing the details surrounding the deaths of 334 people who died during the first 4 weeks of the 1854 epidemic. John Snow, along with his assistant Mr John Joseph Whiting, visited the dwellings of every person who died from cholera in South London during this period. With utmost attention to detail and at great risk to their own personal health, Snow and Whiting recorded important details surrounding the deaths of these individuals. These data provided undeniable evidence that cholera was spread through the ingestion of contaminated water and, as noted by Sir Austin Bradford Hill, are one of the reasons why England and the rest of the developed world have been free from epidemic cholera since the late 1800s. In 1855, John Snow printed 300 copies of his original essay On the Mode of Communication of Cholera at a personal cost of more than £200. He sold only 56 copies. In 1936 Wade Hampton Frost, a professor of epidemiology at the Johns Hopkins School of Hygiene and Public Health, reprinted Snow’s original essay; but he deleted the South London data that Snow collected at such a great personal cost and that were of such great epidemiological value. Frost only listed the first 23 of the 334 entries that John Snow recorded in his original essay, and followed this truncated list with the words: ‘In the original publication the list of deaths is continued in this form for a total of twenty-five pages (p. 139)’. In contrast to the small number of copies that John Snow sold, thousands of copies of the1936 reprint were published and widely disseminated. Unfortunately the wide availability of this reprint has perpetuated the omission of these data and undermines the role that they played in identifying the mode of communication of cholera. To commemorate the 160th anniversary of the publication of Snow’s second edition of On the Mode of Communication of Cholera and to redress this epidemiological slight, we highlight John Snow’s important work in South London, unearth the original data that Snow collected at great risk to his own personal health and present a first-time mapping of these data in time and space. We trust that this piece will foster a deeper appreciation for John Snow’s contribution to epidemiology and increase respect for small yet valuable epidemiological data.