The Ghost Map

Shortly after the pump handle was removed from the Broad Street pump, an engineer named Edmund Cooper began researching the epidemic on behalf to the city’s sewer commission. Cooper wanted to “bury” (as it were) the rumor that ancient corpses had caused the outbreak. Thus, he created a map of the outbreak, superimposed over the city’s sewer lines. Cooper’s map was a milestone in epidemiology (the study of diseases and how they spread), setting the standard for future “dot mapping.” However, Cooper’s map offered too much data, and too many confounding factors in the cholera outbreak.

John Snow then began making his own map of the cholera epidemic. Snow’s map showed each victim’s proximity to the Broad Street pump, emphasizing his waterborne theory. Snow also used icons representing the foot traffic around the pump—implicitly refuting any miasmatic theories that the pump was emitting noxious fumes. To make his map, Snow employed a mathematical tool called the Voronoi diagram. A Voronoi diagram divides a two-dimensional space into distinct spaces, whose common quality is that they consist of points that are closer to a specific, predetermined point than to any other point. For his map, Snow divided London into color-coded regions, based on the region’s closest available water pump.

John Snow’s map was a milestone in the history of epidemiology, reflecting both Snow’s training as a doctor and the painstaking research he conducted in the Soho area between the late-1840s and mid-1850s. However, the map still failed to convince the London Epidemiological Society of Snow’s waterborne theory of cholera. For the time being, the miasma “paradigm” (framework of assumptions or ideas) proved too strong.

Even if John Snow’s map failed to convince the medical establishment, it convinced one very important person of the veracity of the waterborne theory: Henry Whitehead. Had Whitehead not seen Snow’s maps, he might not have been converted to the waterborne theory, and he might not have convinced the St. James Vestry Committee to conclude that contaminated water started the outbreak. The St. James Committee’s conclusion was a decisive step forward in the history of epidemiology, accelerating the public’s adoption of Snow’s theory.

The 1854 cholera epidemic was a horrific episode in London’s history, but it had a silver lining: it was a triumph of science. Snow was a master at drawing bold conclusions from the data. Furthermore, the aftermath of the epidemic represented a triumph of amateur research: Henry Whitehead was a local figure with no particular training, but he used his rapport with Soho locals to assemble crucial data. It’s unclear how much of a personal relationship Snow and Whitehead had with each other—however, it would appear that “a powerful bond formed between them.”

In the years following the outbreak, the waterborne theory of cholera grew more popular, but miasma theory continued to dominate, thanks largely to Sir Benjamin Hall’s authority. However, “the confidence of the miasmatists” took a big hit in 1858, when winds blew the disgusting smell of the Thames through the city. To miasmatists’ surprise, the death rate for London in 1858 was perfectly normal. But just as miasma theory was beginning to collapse, John Snow suffered a stroke, and died days later—he was only forty-five.

In the late 1850s, partly because of Snow’s groundbreaking research, the London municipal government decided to build a new sewer system. London’s sewer system was a triumph of engineering every bit as impressive as the Eiffel Tower or the Brooklyn Bridge; it was also a triumph of urbanism, proving that a city’s population could work together to benefit itself.

In 1866, London experienced its last great outbreak of cholera, largely in its East End. William Farr assembled the evidence and concluded that the vast majority of the victims had been customers of the East London Water Company. He then commissioned notices that all Londoners should boil their water before drinking it. Farr’s actions prompted a city-wide investigation of the East London Water Company’s policies, and it was discovered that eels swam in the company’s water supply. Henry Whitehead was an important figure in the 1866 investigation; he helped uncover a pattern of company negligence that almost certainly resulted in thousands of deaths. Although Snow was dead by this point, Farr and Whitehead regularly credited him for his waterborne theory of cholera. Indeed, Farr (who’d once been a miasmatist) wrote as if he and the medical community had always agreed with Snow. The waterborne theory had at last become the dominant scientific paradigm.

In the 1880s, the great German scientist Robert Koch discovered the Vibrio cholera bacterium, further strengthening the waterborne theory of cholera. Some figures, such as Edwin Chadwick, continued to support miasma theory, but most public health institutions embraced Snow and Koch’s research. In London, the new sewer system resulted in cleaner drinking water and a cleaner Thames. In the early 20th century, there were cholera outbreaks in many Western cities, but these outbreaks almost always prompted the authorities to modernize infrastructure. By the 1930s, cholera had “become an anomaly in the world’s industrialized cities.” However, cholera continues to threaten the developing world.

In the 21st century, the world’s biggest cities contain more than 20 million people. The scavenging classes of Victorian London live on in New Delhi, Dhaka, and other cities. Squatters have built entire communities for themselves, without the help of urban planners or the municipal government. But of course, these squatters face considerable dangers, not the least of which is a lack of clean water. Every year, some two million children die from diseases, like cholera, resulting from a lack of clean water. If the urban planners of the 21st century are to fight these problems, they’ll be dealing with ten times as many people as Farr or Chadwick had to deal with.

Urban planners have proposed ingenious solutions to the problems of disease and disorganization in large cities: small, cheap water purifiers, and generators that run on excrement. Furthermore, the world’s largest cities don’t seem to be on “a collision course with themselves,” as was the case in London in the 19th century. Using the cartographical and epidemiological methods pioneered by John Snow and Henry Whitehead, contemporary thinkers have found ways of reorganizing cities more efficiently. The Internet has also expanded the amount of information available to researchers—gone are the days when Snow and Whitehead had to go door-to-door to learn about the cholera outbreak.

In contemporary times, cities show no signs of disappearing. The modern metropolis may be overcrowded and crime-ridden, but “many people actually like the density of urban environments, precisely because they offer the diversity of Viennese bakeries and art movies.” Furthermore, many cities have become safer, partly because of new information technologies. For instance, New York City introduced a 311 phone service, designed for people who wanted to report information less urgent than the kind usually repotted by calling 911. During the 2003 New York blackout, hundreds of people called 311 to inquire about storing insulin (which is usually supposed to be kept refrigerated). 311 calls alerted the city authorities to a health issue they hadn’t even considered.

The 311 calling system reflects a paradigm for urban planning that emerged thanks to John Snow and Henry Whitehead’s cholera research. The first aspect of this paradigm is the importance of “local experts”—i.e., people like Henry Whitehead with vast, valuable experience with their community. The second aspect of the urban paradigm is the “cross-disciplinary flow of ideas.” Often, the best way to solve an urban problem is to mix different disciplines and people from different professions. John Snow was a true polymath: a mapmaker, chemist, demographer, and physician who embodied the paradigm.

In his later years, Henry Whitehead served as a minister in various northern English cities; he died in 1896. A portrait of John Snow hung in his office—to remind him that “in any profession the highest order of work is achieved … by patient study of eternal laws.” If Whitehead were to walk through the streets of Soho today, most of the things he’d see would be utterly foreign to him. However, the basic spirit of innovation and entrepreneurship hasn’t changed much in the last 150 years. The one building that has remained constant since Whitehead’s time in Soho is the pub at the corner of Cambridge Street, just a few feet from the old Broad Street pump—a pub that’s now called The John Snow.