Prologue
John Snow (1813-1858) was a leading British anesthesiologist who volunteered for several years to conduct his own investigations of the mode of cholera transmission. Snow is best known for his investigations and intervention during a "most terrible outbreak" of cholera that began on August 31, 1854 in the Broad Street, Golden Square section of London.
Due to time limitations, our case study will concentrate on Snow's work in that outbreak. In a nutshell, as the story is often told, Snow observed that the cases of cholera were predominantly among people who had drunk water from a "much frequented" pump on Broad Street. Snow asked for the pump handle to be removed, and the outbreak - which had already begun to decline due to a fleeing populace and the deaths of many in the neighborhood - was terminated.
Actually, the case is much more complex and interesting than that. We must begin with a brief background.
Cholera
Cholera was widely known in the ancient and medieval worlds, but its modern history began in 1817 in India (Longmate, 1966). Scholars divide its subsequent history into at least seven multi-national epidemics (or pandemics) that have swept Africa, Asia, Europe, Central America, North America, South America, and the Middle East in the Nineteenth and Twentieth centuries.
In Britain, the first modern cholera epidemic occurred in 1831-32, causing over 23,000 deaths. This epidemic did not produce an immediate burst in public health legislation from the British government, but it did have the lasting effect of launching the sanitary reform movement. "This [movement] at first embraced the need for hygienic housing, [paved] roads, piped water, efficient sewers and the protection of the public against 'nuisances', but soon came to include much else: public baths and wash-houses, parks and playing fields, and even, on the fringes of the movement, the demand for schools and libraries for the poor" (Longmate, 1966, p.143). This movement was led by Edwin Chadwick, a Minister of Parliament, and several other ardent students and friends of philosopher Jeremy Bentham, whose utilitarianism had taught them that the task of government was to provide the greatest good for the greatest number of citizens. Charged in 1839 with the surveying the sanitary conditions among the poor and working classes, Chadwick and colleagues produced in 1842 an exhaustive and influential tome, The Report on the Sanitary Condition of the Labouring Population of Great Britain. Spurred by this and other reports, the sanitary reform movement prospered and appealed to public opinion. A Public Health Bill and a Nuisances Removal Bill (known unofficially as the Cholera Bill) were ultimately passed in 1848, but only because the arguments of the reformers were amplified by the footsteps of another cholera pandemic approaching from the continent.
Two more British epidemics in 1848 and 1849 brought at least 250,000 cases and 53,000 deaths (Longmate, 1966; Pelling, 1978). It was during these scourges that Snow began his investigations of cholera.
Snow's Prior Work
It is a mistake to begin and end a presentation of John Snow with the Broad Street outbreak, as do many presentations of his work. To begin Snow's work with the Broad Street outbreak makes it appear as if Snow there had a stroke of genius, when instead his studies and intervention were the culmination of years of diligent (and voluntary) research.
Snow's celebrated Broad Street work stood on the shoulders of six years of exhaustive studies of cholera transmission. Snow began these studies after the horrid London cholera outbreak of 1848-49, and never stopped. In the first edition of his now famous book On the Mode of Transmission of Cholera, published in 1849, Snow had already implicated contaminated water as a major contributor to cholera transmission. This made sense to Snow because cholera's major symptoms were intestinal; Snow linked his observations of the biology of the disease to his hypotheses about its spread.
In the summer of 1854, another major cholera epidemic had arrived in London. Snow spent several months that year continuing his ingenious studies that implicated sewage in the River Thames as a major source of the scourge. Some water companies drew their water from contaminated parts of the Thames, Snow observed. By going house to house and by tracing victims, he found that houses that bought their water from those companies were much more likely to have cholera deaths. (Today, epidemiologists reading Snow's brilliant manuscript recognize several well-known methods of epidemiological research that have since been named and refined.)
Thus, by the time the Broad Street outbreak occurred in September of 1854, Snow already had a working hypothesis that water was a major cause of the disease. Rather than being a stroke of genius or only an observation of a statistical association, his Broad Street activities were founded on years of untiring hypothesis-driven research. Snow's second edition of On the Mode of Transmission of Cholera in 1855, documenting these studies, remains an inspiring read.
To end the story with Broad Street is to falsely imply that Snow's intervention there decisively demonstrated the cause of cholera and solved the cholera problem. In the Epilogue, we will see that this was not the case.