Documents that Changed the World
John Snow’s Cholera Map
Narrated by Andrew Brink, MLIS ‘12
Introduction
The year is 1854, and you’ve been sent on a simple errand. Your family needs water, and so you
make your way to one of the neighborhood water pumps—the pump closest to your house. You
fill a bucket with water, and return home. You don’t know it yet, but in choosing to use that
particular pump, you’ve just determined whether your family will live or die.
This is the story of the English physician John Snow and the map he created during a
devastating cholera outbreak in London in 1854.
On the one hand, it’s a complicated story, inhabited by mysterious, microscopic organisms, a
young and crowded metropolis strangled by dung heaps that stood as tall as houses, and
factions of scientists and civic leaders at odds over how to eliminate an invisible killer.
On the other hand, it’s a very simple story.
It’s a story about death. In Victorian London, lives hung in a delicate, unfair balance. Your life,
and the lives of everyone you loved, depended on where you got your drinking water. You lived
or died by the simple choices you made—choices we make fearlessly today. If you chose the
water pump to your right, your family lived. If you chose the water pump to your left, your
family is dead by dawn, their bodies suddenly, mercilessly emptied by cholera.
This is also a story about birth—the origins of the present-day metropolis, the emergence of
modern epidemiology, and the ancestors of John Snow’s cholera maps that foreshadowed
information systems, like GIS, that allow us to analyze, visualize, and understand the patterns
and trends that affect our lives.
Snow’s use of mapping is the contemporary equivalent of Google Maps or social-networking
sites, like Yelp, that allow users to enhance existing maps. Just as we use Google Maps and Yelp
to record where we’ve been and what we’ve experienced, Snow added layers of local and
personal knowledge to available maps in order to tell the terrible story of an epidemic.
Ultimately, this is the story of a document that changed the world by giving voice to a lifesaving idea. Like all good maps, Snow’s map inspired a journey. In illustrating the patterns
created by a devastating epidemic, he opened the eyes and minds that would eventually subdue
the killer named cholera. Snow’s map gave Victorian England exactly what it was looking for—
a way out.
Title
A document that changed the world.
John Snow’s cholera map, published in the 1855 edition of his monograph titled On the Mode of
Communication of Cholera.
The Killer Cholera
Cholera is a terrible way to die. It arrives unseen, unannounced, and then kills with cruel and
baffling speed. Snow’s contemporary, the poet Elizabeth Barrett Browning, lamented cholera’s
ability to “catch” a person with “cramps” and “spasms” that turn gentlemen “dead-blue.” A
clinical description explains the cause of these deadly contractions: cholera is a severe
intestinal infection caused by ingesting food or water contaminated with the cholera bacteria.
The bacteria cause the body to expel a profuse amount of watery diarrhea that leads to a
sudden, lethal state of dehydration. Just imagine: your child, husband, or wife complains of a
stomachache. In the few minutes and hours it takes to fetch relief—the comfort of words,
water, or a doctor—you witness your loved one vanish. Cholera can cause a person to lose up
to thirty percent of their body weight in a matter of hours.
Death is only the beginning for cholera. While cholera demands a lot from its victims, it
requires little to multiply. The discharge caused by cholera is colorless, odorless, and contains a
large amount of the cholera bacteria; all it needs to spread is an environment in which people
frequently ingest other people’s waste.
In 1854, London was the world’s most populous city, with two and a half million people. For all
the marvels it contained—like the Crystal Palace—London lacked public-health departments
and sewage removal technology to effectively manage the waste generated by a city that would
be considered dense even today.
This resulted in overflowing cesspools that forced the river Thames to serve an ill-fated, dual
purpose—acting as both a main source of water and a sewer system. This made John Snow’s
London the perfect host city for an infectious disease.
The Map Itself
So, what does a cholera map look like?
I’m staring at the map John Snow published in 1855—or at least a digital reproduction of it
found on UCLA’s Department of Epidemiology website. This map documents the grim footprint
left in London’s Soho neighborhood during an outbreak that occurred there in 1854.
As with most maps, I see street names and points of interest, like Golden Square and Piccadilly
Circus. I also see hash marks rising, in varying heights, from the straight edges formed by
streets. It’s easy to imagine these lines representing some kind of score, and it’s obvious that,
regardless of who is winning or losing, all the action is emanating from a dot labeled “pump”
sitting at the point where Broad and Cambridge streets meet.
Without knowing anything about cholera or London, I can see that something monumental
happened at the corner of Broad and Cambridge.
Which is true. The water pump sitting at this corner drew its water from a source contaminated
with cholera. The towers of hash marks do represent a score--they tally the lives claimed by
cholera. More than five hundred people died within two hundred and fifty yards of this water
pump within a ten-day period.
At the time, Snow was a noted physician, celebrated for his pioneering work with anesthetics.
Snow had mastered the use of chloroform, and this breakthrough brought much-needed relief
to Londoners undergoing medical procedures. His work was so respected that he was called
upon to comfort Queen Victoria, administering chloroform as she gave birth to her eighth child.
Snow’s professional success allowed him to pursue a personal obsession: solving the riddle
named cholera.
Overload and Clarity
By the time John Snow mapped cholera’s impact on Soho, it had been killing Londoners for
several decades. It wasn’t uncommon for an entire family to be killed by cholera within a twoday period. People were dying, and there were many scientists joining Snow in trying to
answer the anxious question: “why?” Some proposed that cholera was caused by foul air
emanating from London’s graveyards. Others suggested it was linked to atmospheric
conditions. It even seemed possible that the disease was a poison leaking from the earth itself.
These theories were variations on the dominant worldview of the time known as the “miasma”
theory. Miasmists believed that disease was caused by harmful mists and vapors that arose
from filth.
Given how filthy Victorian London was, it’s not surprising that many of the most powerful
people in London—members of Parliament, clergymen, and city commissioners—embraced
the miasma theory.
What is striking is something that can be hard for us to imagine today: in 1854, Londoners
were suffering from information overload. Each theory about cholera had its own proponents,
supported by streams of data recorded and disseminated in newspapers, dot maps,
monographs, and lectures. Victorian England was home to an information revolution, with
demographers recording births, marriages, weather, air quality, and, for the first time, tallying
deaths by cause, location, age, and occupation. If you knew how to assemble the pieces, the
patterns affecting British society—the ebb and flow of birth and death, cause and effect, illness
and prevention—came into view.
John Snow used the tools available to him at the time, assembling the pieces and solving the
mystery of cholera. And he did it with a map.
Map Details and Predecessors
Based on what he learned from studying a previous cholera outbreak in London, Snow
developed a theory that cholera was transmitted by water. He arrived at this theory after
examining the relationship between cholera deaths and sources of water in South London,
where he found that far more people were killed by cholera in households that received water
from a supply that mingled with sewage.
The Soho outbreak struck close to home—Snow lived and worked near the Broad Street water
pump. In creating his famous cholera map, Snow wedded raw data—including the mortality
rates recorded by the local registrar—with street-level knowledge of the patterns and habits of
his neighbors acquired through observations, interviews, and being part of the neighborhood.
Snow knew which pumps individual families were known to drink from, and he knew the
distance between the homes of those who died and the different water pumps they might have
used.
Snow distilled all of this information into graphic representations of life, death, and water
sources. The hash marks, street and building names, and dots noting the location of water
pumps were placed on top of an existing map that had been produced by C.F. Cheffins, a
surveyor and mapmaker.
The result is a map that paints a clear picture of how cholera travels through a community. The
result is a map that scientists, scholars, and writers still, to this day, consult for direction.
Writer Steven Johnson notes that “part of what made Snow’s map groundbreaking was the fact
that it wedded state-of-the-art information design to a scientifically valid theory of cholera
transmission.
Geographer Tom Koch and statistician Edward Tufte both admire the pictorial aspect of the
map, which resulted in placing data in the right context for considering cause and affect. In
being so selective about what information was included—the street names, breweries,
workhouses, and water pumps—the map revealed an overwhelming connection between the
Broad Street pump and cholera transmission. In tallying cholera deaths while also showing the
location of the area’s other water pumps, the Broad Street pump was shown standing at the
epicenter of the epidemic.
This map was born out of Snow’s previous attempts to condense and communicate his
waterborne theory. Snow created a map during his South London study that featured handinked dots, which were hard to read, and cloudy colors that tried, but failed, to show the
connection between cholera deaths and water sources. Snow also published a table to tell the
same story. But it wasn’t quite right. It lacked key pieces of information gleaned later through
interviews and observing the patterns of daily life in Soho.
Snow corrected such errors in producing his 1855 map, which turned out to be a document
that could tell a story that a table or previous maps could not. It was a document that changed
the minds of individuals who went on to change policies that transformed city living from a
death sentence to a sustainable and life-affirming existence.
Given the effect this map has had on urban life, cartography, and science, you may want to
know where the map is today. If any original copies of the map exist, no one seems to know, or
care. Which doesn’t mean the map is hard to find. Authentic copies of the map are found on
university websites, in textbooks, and popular science books. But the whereabouts of an
original copy appears to be unknown. When it comes to determining the value of Snow’s map, it
seems that the information contained in the map is more important than the document itself.
Map Reflection
What is it about a map that enables it to communicate a scientific breakthrough when other
documents or forms of communication—indexes, lectures, monographs—can’t?
What is it about a map that allows it to leave an indelible stamp—a lasting idea—on society
long after the map itself has decayed or been junked, recycled, or lost?
The power contained in a map lies in its ability to provide sure footing in the midst of unknown
territory. In providing direction, a map complements the imagination: it tells us where we are,
where we are going, and where we we’ve been. Snow’s creation demonstrates that maps
provide more than simple directions—they can also provide much-needed clarity.
I’m looking at Snow’s map, and the neighborhood I see is a place that can no longer be visited.
Soho, as represented in this map, does not exist anymore, just as the world depicted in this map
doesn’t exist anymore. Today, Broad is now called Broadwick Street, and Cambridge is now
Lexington Street. Today, we know how to prevent and treat cholera, and how to build healthy,
sustainable cities.
We may not be able to use this map to travel through present-day Soho. Instead, we can use it
for something even more meaningful. We can use this map to return to the exact corner where
the world changed.