The Ghost Map: The Story of London’s Most Terrifying Epidemic—and How It Changed Science, Cities, and the Modern World

“Waste recycling turns out to be a hallmark of almost all complex systems, whether the man-made ecosystems of urban life, or the microscopic economies of the cell.” 

While Johnson’s book is in a broad sense a history, it also encompasses a great deal of science, and many other forms of life besides human life. This sentence, which makes a connection between recycling and the functions of cells, is typical of the scope of this book. 

“‘There is no document of civilization that is not also a document of barbarism.’”

This is a quote from Walter Benjamin, who also provides the epigraph for this book. It refers specifically to the overcrowded pauper graves that existed in Victorian-era London and horrified many Londoners at the time. Johnson suggests that the popularity of the miasma theory—the theory that cholera was transmitted through smell, rather than through swallowing—rested in part on the superstitious revulsion that these mass graves aroused.

“The history of knowledge conventionally focuses on breakthrough ideas and conceptual leaps. But the blind spots on the map, the dark continents of error and prejudice, carry their own mystery as well.”

Johnson’s focus in this book is as much on the underside of Snow’s “breakthrough” theory on cholera as it is on the theory itself. He pays as much attention to the mistaken proponents of the miasma theory, and to the unwitting destruction that they caused, as he does to Snow’s groundbreaking studies. One background thesis in his book is that errors and blind spots are necessary, in order to throw progress into light. 

“[E]pidemics create a kind of history from below: they can be world-changing, but the participants are almost inevitably ordinary folk, following their established routines, not thinking for a second about how their actions will be recorded for posterity.” 

The casualties of the 1854 cholera epidemic will be given their own kind of posterity by Snow’s “ghost map,” which represents the dead as black bars on a city street. In this way, their absence is honored and built into the city landscape. As a part of Snow’s argument about the transmission of cholera, these “ghosts” also become a force for progress and change.

“The idea of microscopic germs spreading disease would have been about as plausible as the existence of fairies to most practicing doctors of the day.”

Snow was able to establish his theory about the transmission of cholera without any of the sophisticated scientific equipment that we have today. His tools were simply doggedness, close observation and an indifference to popular theories (many of which were markedly superstitious).

“It was a kind of haunting, couched in the language of pseudoscience: the dead of one era’s epidemic, returning centuries later, to destroy the settlers who had dared erect homes above their graves.”

Johnson is referring to the Victorian-era theory that the smell of mass pauper graves and the disrespectful closeness of apartment buildings to these graves was partially responsible for cholera epidemics. It is a belief that shows the ease with which fear, superstition, and possibly a degree of societal guilt can be reframed as scientific theory. 

“To live was to be not dead yet.”

Because Victorian-era Londoners had lived through more than one cholera epidemic, their attitude towards such disasters—and towards death itself—was more resigned than that of modern-day society. Johnson compares their stoicism around death to the mass anxiety that many city-dwellers in the States experienced after the attacks of 9/11 and suggests that it is the expectation of safety—rather than its opposite—that makes people more susceptible to anxiety. 

“The metropolitan city, as a concept, was still unproven.”

While this book is a history of a cholera epidemic and the discovery of the origins of cholera, it is also a history of changing attitudes towards cities. Johnson suggests that Snow and Whitehead were not only groundbreaking investigators and scientists, but that they also modeled a new tolerance and urbanity that changed people’s view of great cities such as London for the better.

“Once you get to why, the story has to widen and tighten at the same time: to the long durée of urban development, or the microscopic tight focus of bacterial life cycles. These are causes, too.” 

Johnson is writing here about the myriad causes of cholera, and he is also, in a sense, defending the method of his book. This method is lateral—outwards-spreading and associative—more than it is linear. He is trying to make the case that distant causes matter as much as immediate ones, and that a wider focus on the past helps us to better understand our own present.

“Herein lies the dominant irony of the state of British public health in the late 1840s. Just as Snow was concocting his theory of cholera as a waterborne agent that had to be ingested to do harm, Chadwick was building an elaborate scheme that would deliver the cholera bacteria directly to the mouths of Londoners.”

As president of the Board of Health, Chadwick arranged to have sewage funneled into the Thames river, believing that the smell of it was causing citizens harm. Although Chadwick was at cross purposes with Snow, he also indirectly helped Snow to hone on his theory by concentrating the source of the cholera bacteria. This is an example of the sorts of “blind spots on the map” that Johnson wishes to honor in his book. 

“It’s not just that the authorities of the day were wrong about miasma: it’s the tenacious, unquestioning way they went about being wrong.”

While the proponents of the miasma theory went through a detailed pretense of supporting their theory with empirical evidence and investigations, they started with their theory first. This is one reason the theory survived for as long as it did. This is the converse of John Snow’s way of working, which was to build a theory from his investigations.

“Great breakthroughs are closer to what happens in a flood plain: a dozen separate tributaries converge, and the rising waters lift the genius high enough that he or she can see around the conceptual obstructions of the age.” 

Johnson honors the importance of collaboration and community in his book, whether it is the photosynthesizing collaboration between algae and coral or the collaboration between two disparate humans. He is generally against the idea of a single figure changing history, and here makes the point that Snow—as solitary and eccentric as he was—was also reliant on connections and luck.

“Snow still had a long list of potential converts to win over […] But in the short term, his primary nemesis would be the Reverend Henry Whitehead.”

Johnson sees Snow’s and Whitehead’s eventual collaborative friendship as important precisely because it was so unlikely. It was therefore a new kind of venturesome urban friendship, one based on mutual ambition and intelligence rather than on social common ground.  

“And so the ghosts of the Broad Street outbreak were reassembled for one final portrait, reincarnated as black bars lining the streets of their devastated neighborhood.”

Johnson sees Snow’s ghost map as not only honoring the dead of the 1854 cholera epidemic but as paving the way for a different kind of city map, and therefore a different view of city life. He sees Snow’s ghost map as utilizing both a broad, aerial view of the city—seeing his city from up above and from the distance of history—and a specialized, local one. He suggests that this confluence of views is how we still view big cities today.

“When you combined Snow’s original data with Whitehead’s more exhaustive investigations, when you factored in the index case and the decaying brickwork, the conclusion was inevitable: the pump was the source of the outbreak.”

While Snow and Whitehead built a meticulous case for the Broad Street pump being the source of the 1854 cholera outbreak—and by implication for cholera being transmitted through swallowing rather than through smell—this case still took some time to be accepted by scientists and public health officials. This shows the stubbornness of received opinions, even in an investigative field like science, and the consequent slowness of change.  

“But Broad Street should be understood not just as the triumph of rogue science, but also, and just as important, as the triumph of a certain kind of engaged amateurism. Snow himself was a kind of amateur.” 

Snow’s “amateurism,” Johnson suggests, may have given him an edge over many of the supposed experts of his day. It made him willing to step outside of his own discipline—in his search for the origins of cholera—and all of the dogmas and prejudices that went with it. Statistics, for instance, were as necessary to him in his searches as was his own medical training. So were social skills of the sort that Snow’s friend Henry Whitehead possessed.

“The triumph of twentieth-century metropolitan life is, in a real sense, the triumph of one image over the other: the dark ritual of deadly epidemics replaced by the convivial exchanges of strangers from different backgrounds sharing ideas on the sidewalk.”

Johnson sees Victorian-era London as a dark and fearful city, with a dark and fearful image of itself. He sees Snow and Whitehead, in helping to stop cholera epidemics, as also helping to spread a more positive image of metropolitanism.  

“If Snow and Whitehead’s Broad Street investigation showed that urban intelligence could come to understand a massive health crisis, Bazalgette’s sewers proved that you could actually do something about it.” 

Bazalgette designed the London sewer system that still exists today; he is therefore as important a figure as Snow or Whitehead. His presence in this book helps to prove Johnson’s thesis that no single person brings about a revolution, but that it is rather a gradual and halting process brought about by many.

“Farr’s conversion to Snow’s doctrine was so complete that he literally rewrote history to make it appear as though Snow’s ideas had more initial success than they had actually enjoyed.”

The statistician William Farr was initially opposed to Snow’s theories, even while he helped him out in his investigations. That he later changed his mind so completely is an illustration of a very human sort of self-deception. It also shows how completely we tend to take current theories and doctrines for granted and to assume that they have always existed. 

“Squatter cities lack most of the infrastructure and creature comforts of developed metropolitan life, but they are nonetheless spaces of dynamic economic innovation and creativity.”

Johnson draws a parallel between the Victorian-era waste scavengers of London and the squatter city dwellers of today. He sees both groups as urban underclasses that both shadow middle-class hierarchies and come up with their own innovative ways of coping with their disenfranchisement. 

“The irony, of course, is that digital networks were supposed to make cities less attractive, not more. The power of telecommuting and instant connectivity was going to make the idea of densely packed urban cores as obsolete as the walled castle-cities of the Middle Ages.”

Johnson suggests that the appeal of big metropolitan cities is finally more enduring than the appeal of “connective” technology, to the point where this technology has been put to the use of developing city networks. This is the case with Google maps, for example, as well as with other collaborative internet maps in which city dwellers describe their own neighborhoods.

“Indeed, it is the peculiar nature of epidemic disease to create terrible urban carnage and leave almost no trace in the infrastructure of the city.” 

Snow’s ghost map, as well as Johnson’s book, aim to restore visibility to this invisible infrastructure: the casualties of epidemics. Both suggest that these “ghosts” are as present in city landscapes as are traditional historical monuments. 

“The history books tend to orient themselves around nationalist story lines: overthrowing the king, electing the presidents, fighting the battles. But the history book of recent Homo sapiens as a species should begin and end with one narrative line: We became city dwellers.”

This book’s focus is finally global more than it is local, which is to say that it examines the 1854 London cholera epidemic as a way of examining cities and city living in general. Johnson sees the world as having grown increasingly metropolitan since Victorian times. He also sees the 1854 cholera epidemic as anticipating many of the challenges that city dwellers must face today.