Review: Steven Johnson, The Ghost Map

John Snow’s map of the cholera dead after London’s 1854 epidemic is often heralded as one of the earliest examples of graphical data analysis. Steven Johnson’s The Ghost Map gives a lot of background about the London of the 1850s, Snow’s work, and how central the map really was.

London and the Cholera

John Snow actually made his name by refining the fledgling field of anesthetics using chloroform and ether from a hit-and-miss operation to a reproducible science. He did that based on then relatively recent research on the behavior of gases and how it depends on temperature (which is important for dosage).

Snow had developed and published a theory of how the cholera spreads through water, but had not been able to convince the medical establishment at the time. He was already working on a major study of the water supply in South London and its impact on people’s health (some of that water came from clean sources, some of it from the polluted Thames downstream from London’s core) when people started dying rapidly on and around Broad Street, close to where he lived. That such a forceful outbreak occurred so close to him was coincidence, but he was prepared and saw the chance to apply and prove his theory.

It is easy today to look down on theories about disease like the miasma (essentially the smell of human waste), but Johnson paints such a vivid picture of the smells, the living conditions (London at the time had a greater density of people than Manhattan has today), and the general lack of knowledge about what caused disease, that it becomes possible to appreciate how difficult it was to break out of that kind of thinking.

In addition to Snow, Johnson gives a lot of credit to Rev. Henry Whitehead, who was the parish priest of the area. While initially skeptical, Whitehead was open to evidence and changed his mind when Snow presented him with a clearly argued case for cholera’s waterborne nature. Whitehead then helped track down many of the survivors who had fled, to show that they had not drunk from the tainted well (an important part of the overall argument), and even looked for and found the index case that had started the entire outbreak.

The Map

Given the title of the book, I expected to read more about the famous map. But it turns out that not only was the map not a crucial analysis tool, others had actually drawn similar maps of the outbreak before Snow. For all we know, Snow only drew a map long after the end of the outbreak to illustrate his argument in a report and in the second edition of his monograph on the cholera. The famous map served as a communication tool, rather than as a way to discover the cause of the outbreak.

The book demonstrates in great detail the many steps it took for Snow to come up with his theory, collect evidence, perform experiments, etc., in order to get to the point where he knew what he was looking for when the 1854 outbreak occurred. Snow’s great achievement is not merely drawing a map, but developing a whole new way of thinking about how disease spreads. The map was a tool, and it helped him make his point (especially the version he drew that included a kind of Voronoi diagram to show who lived closer to which water pump).

The Book

Several times, Johnson argues that the ability to understand the spread of disease, and ability to fight it, is key to building dense, large-scale cities that house millions of people. While I don’t disagree on that point, he takes that way too far, away from John Snow and cholera to biological and nuclear terrorism. The conclusion already makes that point and gets a bit repetitive. But the epilogue is where it really starts hurting the book. Johnson seemingly tries to make the case for he importance of what happened in the 1850s in London for the present and the future. But it ends up feeling forced and unnecessary. I would recommend skipping the epilogue entirely.

Besides a bit of repetitiveness here and there, as well as the epilogue, it’s an excellent book. Johnson is a fantastic writer, and clearly understands the history as well as the science of what he’s writing about. In an appendix, he explains his use of historic materials (dialogue is only used where he has sources), and what freedoms he took in writing the story. If you care about context, not just a single map on a pedestal, this book will give you a lot of insight into the world it came from, and the revolution in thinking it embodies.

Steven Johnson, The Ghost Map. Riverhead Trade, 320 pages (paperback, also available as hardcover, audiobook, and on Kindle), 2007.

Johnson has a website with several videos of him talking about topics from the book. A fantastic source on the map is also Ralph R. Frerichs’s website on John Snow, despite the rather horrific navigation (the little squares are the links, not the text).