Today marks the 45th installment in a series of articles by HumanProgress.org titled Heroes of Progress. This bi-weekly column provides a short introduction to heroes who have made an extraordinary contribution to the well-being of humanity. You can find the 44th part of this series here.

This week, our hero is John Snow. Snow was a 19^th century English physician, and is considered by many the father of epidemiology. Following a series of cholera outbreaks in London, Snow was the first person to use maps and data records to track the spread of a disease back to its source. Snow’s work provided a foundation for the science of epidemiology. As such, he improved the way humanity confronts public health emergencies.

Snow was born on March 15, 1813 in York, England. His father was a coal-yard laborer and Snow was the oldest of eight siblings. Snow was raised in one of the city’s poorest neighborhoods. As a child, he was exceptionally bright, with a strong aptitude for mathematics. His mother, recognizing Snow’s academic talents, used a small amount of money that she inherited to send him to a nearby private school.

Snow excelled at school. In 1827, aged 14, Snow attained an apprenticeship under Dr. William Hardcastle in Newcastle — about a hundred miles away from York. In 1831, a cholera epidemic began to spread across Europe. By 1832, a nearby coal-mining village was severely afflicted with the disease. With Hardcastle overwhelmed by patients, Snow was sent out to the village to attempt to treat the victims.

Cholera causes its victims to suffer from severe diarrhea and vomiting, which leads to rapid dehydration. It can be fatal within just a few hours. Unfortunately, there was little that Snow could do to help the cholera-stricken miners. The typical medical treatments of the day included laxatives, opium, brandy, and peppermint, were all hopelessly ineffective. A few months later, the epidemic ended. In total, the outbreak killed more than fifty-thousand Britons. The early experience of helplessness in combatting cholera left a significant impact on Snow.

Later in 1832, Snow began to work as an assistant to a colliery surgeon in Country Durham. In 1836, he enrolled in the Hunterian School of Medicine in London. A year earlier, Snow signed an alcohol abstinence pledge and became a supporter of teetotalism. He also became a vegetarian and would only drink water that had first been boiled, so that it was “pure.”

In 1837, Snow started working at the Westminster Hospital and a year later he was admitted as a member of the Royal College of Surgeons of England. In 1844, Snow received his Doctorate in Medicine from the University of London. After graduation, Snow began to work as a surgeon and general practitioner.

For several years, Snow meticulously studied the effects of different anesthetics. In Snow’s day, it was typical for a surgeon to use either too little anesthetic and the patient would wake up mid-surgery, or too much anesthetic, which could cause death. Snow was one of the first physicians to study and calculate the dosages of ether and chloroform needed for use in surgery. For many years, Snow tested the effects of ether and chloroform on himself. By taking notes on how long he was unconscious after different dosages, he was eventually able to work out the optimal amount of anesthetic that patients could handle.

After creating an anesthetic inhaler and publishing his findings in 1847 in a textbook titled On the Inhalation of the Vapour Ether in Surgical Operations, Snow quickly gained fame as Britain’s most accomplished anesthetist. Snow’s stardom eventually led him to administer chloroform to Queen Victoria during the birth of her last two children, Prince Leopold and Princess Beatrice.

Despite Snow’s impressive accomplishments in the field of anesthesiology, his most important work came a few years later, following a series of cholera outbreaks in London. In the mid-19^th century, most physicians thought that diseases such as cholera or the plague were caused by “miasmas” or air pollution. The germ theory of disease had yet to be developed, but Snow theorized that diseases were likely caused by invisible tiny parasites.

In 1848, a new outbreak of cholera struck London and Snow decided to track the disease to its source to find out how it spread. After examining many patients, Snow realized that their first symptoms had nearly always been digestive problems. Snow theorized that the disease must have been ingested through food or water. If the disease had been spread from polluted air, as the supporters of the miasmatic theory believed, then the first symptoms should logically appear in the nose or lungs.

Moreover, Snow reasoned that the severe diarrhea, which was caused by cholera, might be the mechanism by which the germs spread. Put differently, if dangerous germs were present in the diarrhea and the diarrhea contaminated the water supply, the germs could then spread to countless new victims. In 1849, Snow decided, at his own expense, to publish a pamphlet that outlined his thoughts on how cholera was spread. It was titled On the Mode of Communication of Cholera. Although Snow’s work had little effect on his colleagues’ thinking, he pushed on with his research regardless.

In August 1854, another cholera outbreak occurred in the Soho neighborhood in London. Snow found that, of the seventy-three cholera victims, sixty-one had drunk water from a single pump that was located on Broad Street. Snow’s microscopic examination of the water from the Broad Street pump proved inconclusive. Undeterred, Snow plotted the number and location of cholera cases on maps of the area, to highlight the correlation between cholera infections and use of the Broad Street pump. The following month Snow showed his evidence to the authorities and recommended that they remove the pump’s handle, so that no more water could be drawn from the infected source. Although the authorities were not convinced by Snow’s argument, they obliged. Subsequently, the local cholera outbreak quickly ended.

Researchers would later discover that the Broad Street well had been dug just 3 feet away from an old cesspit, which had begun to leak fecal bacteria – a discovery that helped lend credence to Snow’s theory.

Later in 1854, Snow began another project that he called his “Grand Experiment.” Snow began to compare the cholera death rates from households that had their water supplied by two different companies: the Southwark & Vauxhall Company, and the Lambeth Company.

Snow found that the Southwark & Vauxhall company relied on water from sewage-polluted sections of the River Thames. In contrast, the Lambeth Company used water from inlets in the upper Thames – which was located miles from urban pollution. Snow created dot maps and used statistics to highlight the correlation between the quality of the water supplied to different households and incidences of cholera.

Snow enlarged his original 1848 pamphlet into a larger book that included intricate details of all his studies. In 1855, he published the second edition of On the Mode of Communication of Cholera. Today, Snow’s studies are considered to be major events in the history of public health, as it was the first time that maps and data records were used to accurately track the spread of a disease back to its source. As such, many consider Snow’s 1855 book as the foundation of epidemiology. However, despite the work’s historical significance, Snow’s critics and public health officials remained unimpressed, arguing that the enormous quantity of water in the Thames was large enough to dilute any waterborne poison.

Snow’s foresight wasn’t truly appreciated until the 1860s, when our 19^th Hero of Progress, Louis Pasteur, successfully proved the germ theory of disease. Unfortunately, Snow never got to see his work become widely accepted, since he died from a stroke on June 16, 1858. He was just 45 years old.

John Snow was one of the great physicians of the 19^th Century. During his short life, he wrote over a hundred books, pamphlets and essays on a variety of medical topics. He is widely considered to be the founder of epidemiology. Snow’s methods have been copied all over the world and have been used to quell, or at least slow down, a number of potentially-catastrophic outbreaks of deadly diseases, thus likely saving untold millions of lives. For these reasons, John Snow is our 45th Hero of Progress.