Steven Johnson's new series How We Got To Now starts on BBC2 at 7.35pm on Saturday 14th Feb . Here he tells us more about a story featured in the first episode. The book accompanying the series is available now from Penguin .

In 1897, a deadly outbreak of typhoid erupted in the town of Maidstone, in Southeast, England. At its peak, 900 people contracted the disease in a two week period; by the end, 143 of them would be dead. Thanks to the pioneering work of John Snow, Robert Koch, and Louis Pasteur, the public health authorities understood that the outbreak was due to bacterial contamination of the water supply. Thanks to new microscopes developed by the German lens crafters Zeiss Optical Works, they could actually see and even measure the bacterial count in the drinking water.

In earlier outbreaks, the authorities had simply warned people to boil their water before drinking it. But of course there was no way to boil an entire reservoir. But a new idea for fighting the bacteria had emerged: killing the microbes not with heat, but with poison. A Cambridge pathologist named G. Sims Woodhead treated the water with calcium hypochlorite, the potentially lethal chemical that is better known as chlorine, also known at the time as “chloride of lime.” His temporary experiment marked the beginning of a practice that would eventually spread around the world, and would prove to be one of the most significant public health innovations of the twentieth century: chlorinated drinking water.

But Woodhead’s innovation was an emergency measure, instituted temporarily to stop a deadly outbreak. The idea of chlorinating water by default would take another 10 years to develop, thanks to the bold — some might even say reckless — actions of a New Jersey doctor named John Leal.

Like John Snow before him, Leal was a doctor who treated patients but who also had a passionate interest in wider issues of public health, particularly those concerning contaminated water supplies. It was an interest born of personal tragedy: His father had suffered a slow and painful death from drinking bacteria-infested water during the Civil War. His father’s experience in the war gives us a compelling statistical portrait of the threat posed by contaminated water and other health risks during this period. Nineteen men in the 144th Regiment died in combat, while 178 died of disease during the war.

Leal experimented with many techniques for killing bacteria, but he began to focus his energies on chlorine around the time of the Maidstone outbreak. The chemical had already been in wide circulation as a public health remedy. Houses and neighborhoods that had suffered an outbreak of typhoid or cholera were routinely disinfected with the chemical, an intervention that did nothing to combat waterborne disease. But the idea of putting chlorine in water had not yet taken hold. The sharp, acrid smell of chloride of lime was indelibly associated with epidemic disease in the minds of city dwellers throughout the United States and Europe. It was certainly not a smell that one wanted to detect in drinking water. Most doctors and public health authorities rejected the approach. One noted chemist protested, “The idea itself of chemical disinfection is repellent.” But armed with tools that enabled him to both see the pathogens behind diseases such as typhoid and dysentery, and measure their overall presence in the water, Leal became convinced that chlorine — at the right dosage — could rid water of dangerous bacteria more effectively than any other means, without any threat to the humans drinking it.

Eventually, Leal landed a job with the Jersey City Water Supply Company, giving him oversight of seven billion gallons of drinking water in the Passaic River watershed. This new job set the stage for one of the most bizarre and daring interventions in the history of public health. In 1908 the company was immersed in a prolonged legal battle over contracts (worth hundreds of millions of dollars in today’s money) for reservoirs and water-supply pipes they had recently completed. The judge in the case criticized the firm for not supplying waste that was “pure and wholesome” and ordered it to construct expensive additional sewer lines designed to keep pathogens out of the city’s drinking water. But Leal knew the sewer lines would be limited in their effectiveness, particularly during big storms. And so he decided to put his recent experiments with chlorine to the ultimate test.

In almost complete secrecy, without any permission from government authorities (and no notice to the general public), Leal decided to add chlorine to the Jersey City reservoirs. With the help of engineer George Warren Fuller, Leal built and installed a “chloride of lime feed facility” at the Boonton Reservoir outside Jersey City. It was a staggering risk, given the popular opposition to chemical filtering at the time. But the court rulings had severely limited his timeline, and he knew that lab tests would be meaningless to a lay audience. “Leal did not have time for a pilot study. He certainly did not have time to build a demonstration-scale facility to test the new technology,” Michael J. McGuire wrote in his account, “The Chlorine Revolution.” “If the chlorine of lime feed system lost control of the amount of chemical being fed and a slug of high chlorine residual was delivered to Jersey City, Leal knew that would define the failure of the process.”

It was the first mass chlorination of a city’s water supply in history. Once word got out, however, people deemed Leal as a madman or some kind of terrorist. Drinking a few glasses of calcium hypochlorite could kill you, after all. But Leal had done enough experiments to know that very small quantities of the compound were harmless to humans but lethal to many forms of bacteria. Three months after his experiment, he was called to appear in court to defend his actions.

Throughout his interrogation, he stood strong in defense of his public health innovation:

Q: Doctor, what other places in the world can you mention in which this experiment has been tried of putting this bleaching powder in the same way in the drinking water of a city of 200,000 inhabitants? A: 200,000 inhabitants? There is no such place in the world. It has never been tried. Q: It never has been. A: Not under such conditions or under such circumstances but it will be used many times in the future, however. Q: Jersey City is the first one? A: The first to profit by it. Q: Jersey City is the first one used to prove whether your experiment is good or bad? A: No, sir, to profit by it. The experiment is over. Q: Did you notify the city that you were going to try this experiment? A: I did not. Q: Do you drink this water? A: Yes, sir. Q: Would you have any hesitation about giving it to your wife and family? A: I believe it is the safest water in the world.

Ultimately, the court case was settled with near complete victory for Leal. “I do there find and report,” the special master in the case wrote, “that this device is capable of rendering the water delivered to Jersey City, pure and wholesome ... and is effective in removing from the water ... dangerous germs.” Within a few years, the data supporting Leal’s daring move had become incontrovertible: Communities such as Jersey City that enjoyed chlorinated drinking water saw dramatic decreases in waterborne diseases like typhoid fever.

At one point in cross-examination during the Jersey City trial, the prosecuting attorney accused John Leal of seeking vast financial rewards from his chlorine innovation. “And if the experiment turned out well,” he sneered, “why, you made a fortune.” Leal interrupted him from the witness box with a shrug, “I don’t know where the fortune comes in; it is all the same to me.” Unlike others, Leal made no attempt to patent the chlorination technique that he had pioneered at the Boonton Reservoir. Any water company that wished to provide its customers with “pure and wholesome” water was free to adopt his idea. Unencumbered by patent restrictions and licensing fees, municipalities quickly implemented chlorination as a standard practice across the United States and eventually around the world.

About a decade ago, two Harvard professors, David Cutler and Grant Miller, set out to ascertain the impact of chlorination (and other water filtration techniques) between 1900 and 1930, the period when they were adopted across the United States. Because extensive data existed for rates of disease and particularly infant mortality in different communities around the country, and because chlorination systems were rolled out in a staggered fashion, Cutler and Miller were able to get an extremely accurate portrait of chlorine’s effect on public health. They found that clean drinking water led to a 43 percent reduction in total mortality in the average American city. Even more impressive, chlorine and filtration systems reduced infant mortality by 74 percent and child mortality by almost as much.

It is important to pause for a second to reflect on the significance of those numbers, to take them out of the dry domain of public health statistics and into the realm of lived experience. Until the 20th century, one of the givens of being a parent was that you faced a very high likelihood that at least one of your children would die at an early age. What may well be the most excruciating experience that we can confront — the loss of a child — was simply a routine fact of existence. Today, in the developed world at least, that routine fact has turned into a rarity. One of the most fundamental challenges of being alive — keeping your children safe from harm — was dramatically lessened, in part through massive engineering projects, and in part through the invisible collision between compounds of calcium hypochlorite and microscopic bacteria. The people behind that revolution didn’t become rich, and very few of them became famous. But they left an imprint on our lives that is in many ways more profound than the legacy of Edison or Rockefeller or Ford.