John Snow's Map

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John Snow's Map

Epidemiological Investigation Solves London Epidemic


By: John Snow

Date: 1854

Source: John Snow. Map 1, in On the Mode of Communication of Cholera. 2nd. ed. London: 1855. Available online at: Ralph R. Frerichs, MD. UCLA. Department of Epidemiology /School of Public Health. "John Snow's Map 1" (Broad Street Pump Outbreak, 1854). 〈〉 (accessed October 30, 2005).

About the Author: John Snow (1813–1858) was an English physician who made great advances in the understanding of both anesthetics and the spread of disease, especially cholera.


Although the Asian form of cholera may have existed in India since 400 CE, it was not until the nineteenth century that it was known in Europe. The first pandemic, which reached Great Britain in 1831, caused as much fear and panic as tuberculosis did in the early twentieth century and HIV/AIDS does today. The death rate from cholera was over 50 percent and medical opinion was sharply divided as to the cause. At that time, John Snow was a doctor's apprentice gaining his first experience with the disease, noting its symptoms of diarrhea and extreme dehydration.

The germ theory of disease, which holds that viruses and bacteria are the causative infectious agents of diseases such as yellow fever, smallpox, typhoid, cholera, and others, was in its infancy at this time. Some doctors believed in contagion, in which disease spreads from one person to another. Others assumed that miasmata, or toxins in the air, spread disease. Given the dire state of personal and public hygiene in the mid-nineteenth century, either theory held merit at the time.

Snow first began a serious scientific investigation of cholera transmission during the 1848 London epidemic. In his classic essay, On the Mode of Communication of Cholera, published on August 29, 1849, he postulated that polluted water was a source of cholera—especially water contaminated by the waste of an infected person, a not-uncommon occurrence at the time. When an outbreak erupted a few years later in central London at the end of August 1854, close to where Snow himself lived, he resumed his research. The map, published in 1855 as a second edition of his earlier pamphlet, shows how he linked the local cases, shown as dark blocks, to their source—a water pump in Broad Street.



See primary source image.


John Snow's research, detailed on his famous map, is an early example of epidemiology, the scientific study of the occurrence, distribution, and control of disease. Snow's map traced the 1854 cholera outbreak to the Broad Street pump. However, the claim that he removed the pump handle himself—which would, of course, have stopped exposure to the contaminated water—may be false. He recommended its removal, but this was probably done by the local curate, Henry Whitehead, several days after the outbreak began and when, according to collected data, it was already on the wane.

It is partially thanks to John Snow's work in the Broad Street area that Britain suffered fewer major outbreaks of cholera after this time. An influential figure in medical circles, he had been elected president of the Medical Society of London in 1855. Fortunately for British public health, the successful proof of his theory on the transmission of cholera—from person to person via contaminated water—took hold, and the "environmental" theory eventually died away. Although the actual causative agent, the bacterium Vibrio cholerae, would not be identified until 1883, Snow's preventive methods worked. Indeed, they are still effective today, for despite the advent of vaccination and antibiotics, handwashing and the avoidance of contaminated food and water are still fundamental ways of preventing infection.

Because Snow based his investigation on the idea of germ theory, which French microbiologist Louis Pasteur (1822–1895) would later prove, he used a scientific approach and epidemiological study of cholera victims to validate his hypothesis. As his case notes amply demonstrate, much of his research was driven by his patients' visible suffering.

Today epidemiologists work in a similar, although more sophisticated, manner. As causes of most infections are now understood, today's puzzles include chronic conditions such as diabetes, cancer, and heart disease. Infection claimed so many lives in Snow's day that there was hardly a chance to investigate these longer-term medical problems. The twenty-first century equivalent of John Snow's Broad Street pump are lifestyle and genetics, which current research indicates are the major cause of chronic disease. The careful scientific approach that produced John Snow's map still inform the best kind of epidemiology today, with enormous potential to improve human health.



Shephard, David A. E. John Snow: Anaesthetist to a Queen and Epidemiologist to a Nation. Cornwall, Prince Edward Island, Canada: York Point, 1995.

Web sites

UCLA Department of Epidemiology. School of Public Health. "John Snow." 〈〉 (accessed November 11, 2005).

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John Snow's Map

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