"It is August 1854, and London is a city of scavengers."

Thus begins 'The Ghost Map', Steven Johnson's brilliant book on the London cholera outbreak of 1854. The book, on the one hand, is a medical thriller that follows its two protagonists – the physician John Snow and clergymen Henry Whitehead -  as they use reason and evidence to overturn the prevailing orthodoxy on the understanding of communicable diseases and in the process, found the science of modern epidemiology in what is still considered a seminal event in the field of public health.

Simultaneously, the book is also remarkable for its view of the epidemic as a consequence of urbanisation and the response to it as one of the defining milestones that made long-term urbanisation and high density growth sustainable prospects. In making this argument, Johnson starts off by providing us a visceral portrait of London circa 1854, to lay out the background on which the story unfolds. Sample this edited fragment about the scavenger economy of London:

"Just the names [of the scavengers] alone read like some zoological catalogue: bone-pickers, rag-gatherers, pure-finders, dredgermen, mud-larks, sewer-hunters, dustmen, night-soil men, bunters, toshers, shoremen. They were the London underclasses, at least a hundred thousand strong. So immense were their numbers that had the scavengers broken off and formed their own city, it would have been the fifth-largest in all of England…

Early risers […] would see the toshers wading through the muck of low tide, […] their oversized pockets filled with stray bits of copper recovered from the water's edge. […] Beside them fluttered the mud-larks, often children, dressed in tatters and content to scavenge all the waste that the toshers rejects as below their standard: lumps of coal, old wood, scraps of rope.

Above the river, in the streets of the city, the pure-finders eked out a living by collecting dog-shit […] while bone-pickers foraged for carcasses of any stripe. Below ground, in a growing network of tunnels beneath London's streets, the sewer-hunters slogged through the flowing waste of the metropolis. Every few months, an unusually dense pocked of methane gas would be ignited by one of their kerosene lamps and the hapless soul would be incinerated twenty feet below ground, in a river of raw sewage.

London's underground market for scavenging had its own order of rank and privilege, and near the top were the night-soil men […] City landlords hired the men to remove the "night soil" from the overflowing cesspools of their buildings."

With a population of 2.4 million people as per the 1851 census, and grossly inadequate infrastructure, London was "drowning in its own filth". Great numbers of people living closer to each other in unsanitary conditions – the ingredients for the spread of cholera bacterium were optimal.

A growing city with infrastructure unable to keep pace with population, generating tremendous quantities of waste and sewage, and scavenger economies built around the waste – these are themes that resonate across many developing country metropolises even today. London's transformation to a global city with good quality infrastructure therefore can be seen as a beacon of hope for these cities as they confront their infrastructure and public service challenges, but with the added complexity in most cases of much higher populations than London in 1854– in some cities like Mumbai and Delhi, an order of magnitude greater. However, the technologies and solutions available at present to tackle civic problems at scale also provide these cities tremendous opportunity to drive change much faster than London in 1854 was able to.

Having established that London in 1854 was rife with conditions for the spread of cholera, Johnson then describes the untiring work of Snow and Whitehead in fighting against the prevailing orthodoxy of the 'miasma' theory, which held that diseases such as cholera were caused by bad air and smells. Based on their scientific approach of collecting data on where deaths had occurred, mapping this data and comparing this with areas which were considered to be ridden by 'miasma', they were able to conclusively prove that the 'miasma' theory could not explain the deaths caused by the epidemic. They were also able to prove that it was the water from the pump located at Broad Street that caused the spread of the disease. The maps developed by Snow were critical in clinching the case for the water-borne theory.

"When Snow set out to do the second version of the map was to create a Voronoi diagram using the thirteen pumps as points. He would diagram a cell that showed the exact subsection of addresses on the map that were closer to the Broad Street pump than they were to any other pump. But these distances were to be calculated on foot-traffic terms, not the abstract distances of Euclidean geometry. […]

And so the second version of the map […] included a slightly odd, wandering line that circumscribed the centre of the outbreak, roughly in the shape of a square with five or six areas jutting out, like small peninsulas, into the surrounding neighbourhood. This was the area encompassing all those residents for whom the quickest trip for water was to the Broad Street pump. Superimposed over the black bars that marked each death, the amorphous shape took on sudden clarity: each peninsula extended out to embrace another distinct cluster of deaths. Beyond the circumference of the cell, the black bars quickly disappeared."

Modern epidemiology was born out of this map. What is striking is the fact that the physical mapping of the spread of an outbreak enabled the detection of its source. Today, we see wide application of maps in understanding many different types of urban issues, and this owes in substantial measure to John Snow's maps of the cholera outbreak.

Additionally, Johnson also argues that the very nature of 'urban' London was crucial to Snow and Whitehead making their breakthrough.

"And it was precisely [Snow and Whitehead's] metropolitan connection that made this solution possible: two strangers of different backgrounds, joined by circumstance and proximity, sharing valuable information and expertise in the public space of the great city. The Broad Street case was certainly a triumph of epidemiology, and scientific reasoning, and information design. But it was also a triumph of urbanism"

The lasting consequence of the cholera outbreak Johnson posits is the fact that public health authorities oriented themselves towards the new science. What this meant was that there was a new focus on creating infrastructure that enabled the supply of clean water and the removal of sewage to create sanitary, hygienic cities. In the long view, Johnson argues, it is this change that has enabled (and will continue to enable) the trends of increasing urbanisation and denser cities while ensuring health and safety for citizens.

"Establishing sanitary water supplies and waste removal systems became the central infrastructure projects of every industrialised city on the planet. […] The changes ushered in by the sewer system were manifold: fish retuned to the Thames; the stench abated; drinking water became markedly more appetizing. But one change stood out above all the others. In all the years since Henry Whitehead helped track down the Old Ford reservoir contamination in 1866, London has not experienced a single outbreak of cholera. The battle between metropolis and microbe was over, and the metropolis had won."

By Anand Sahasranaman, IFMR Finance Foundation . This post is part of our "Cities in Books" blog series.