A calculated guess by a doctor nearly two centuries ago continues to influence the world today.

Words: Dr Roderick Mulgan  Illustration: Anna Crichton

In 1854, cholera broke out in Soho, London. It was a desperately familiar scenario, well-known across Europe, particularly after cities became crowded in the wake of the industrial revolution. People would start falling ill and dying, and the authorities could do nothing but hope it would play out before too many lost their lives. Nobody understood what it was, and nobody knew what to do. Insight into germs awaited Louis Pasteur, years away.

But this outbreak was different and destined to change medical history. It was quietly observed by a doctor called John Snow, who had an idea. His colleagues believed in miasma, or contaminated air, and he didn’t know one way or the other whether that was right (actually, nobody did). But he thought of how he might find out.

John Snow obtained a map of Soho. He knew from the city authorities which households had deaths, and he marked them on his map. When he was done, he had a cluster, and in the cluster’s centre was Broad Street. More particularly, the water pump on Broad Street.

Before reticulated water, city suburbs did what villages had always done and sank deep holes into the subterranean water table, which became the water source for the surrounding houses. People took their buckets to the communal pump when they wanted some.

“It’s the water pump on Broad Street,” said John Snow.

“Ridiculous,” said everyone else. “Water can’t make you sick.”

But Dr Snow persisted and met the Board of St James’ Parish, which owned the pump, on the evening of Thursday, 7 September 1854, and argued his case. The following day, the handle of the pump was removed, and the disease dissipated.

Modern historians consider the outbreak may have been declining anyway, but John Snow’s decisive thinking has changed the world. Removing the handle of the Broad Street water pump is widely credited with two ideas reverberating to modern times.

The first is the precautionary principle. Dr Snow was not saying the pump was obviously responsible. He was saying it was a viable theory that should be tested.

This simple change of emphasis has powerful implications. It takes decades and considerable cost to know for certain the true dimensions of a threat, and much avoidable harm can accumulate. The precautionary principle says you don’t have to have certainty before you act. Genetic modification is a modern example.

There is plausible concern that modified organisms could damage the environment, and that is enough to justify rigid controls. By the time we know more, it will be too late to act if the risk proves true. Genies do not go back into bottles.

I wrote recently about how insect numbers have plummeted worldwide in the past two decades and the solid circumstantial case against neonicotinoid insecticides. The precautionary principle says banning them should not wait on proof; the remaining insects may be gone when we have it.

Likewise, climate change, overfishing the sea, artificial intelligence and sending probes into deep space to introduce ourselves to aliens. If these go wrong, they will go very wrong indeed. Do we really want to find out the hard way?

Like most big ideas, the precautionary principle has multiple parents, but many historians cite Dr Snow’s quiet determination to disable the Broad Street water pump without any proof of whether it might work as a pivotal moment in getting the idea established.

The other earthquake to emerge from this episode is the discipline of field epidemiology. John Snow had a map and a pen. Today his academic descendants number in the tens of thousands and crunch data in mainframes, but their task is remarkably similar. They are looking for some pattern in the real world and acting in advance of other doctors explaining the mechanisms.

Once again, many other thinkers contributed, but the elegant simplicity of Dr Snow’s reasoning was pivotal.

It is well known today that smoking gives you cancer and a whole range of other nasties, but it wasn’t always. For the first half of the 20th century, doctors thought tobacco smoke was benign. Lung cancer was blamed on fumes from car engines, particularly as cases were seen to rise as cars became more common, starting in the 1920s.

We now know this was just a coincidence, but simply cutting open cancers would never have explained it. The mystery could only be unlocked by taking a group of people with lung cancer and a group without cancer, similar in other respects, such as age and lifestyle, and comparing the incidence of smoking between them. The research, undertaken by a smoker, came out in the 1950s. Tobacco has been on the back foot ever since. This sort of research is painstaking detective work that takes years to get anywhere, but it is the source — and the only viable source — of some of the most useful things doctors know.

Take the effects of high blood pressure. Strokes and heart attacks come from fatty bulges in the walls of crucial arteries that develop over decades and wind up bursting. People with elevated blood pressure have more of them, but until the 1950s, doctors thought that was all good; a natural adaptation to keep blood moving while the pipe got narrower. Epidemiology put them right; high blood pressure makes you more likely, not less, to have a disastrous blockage. It is a cause, not a consequence, and since the epidemiologists pinned that down, a considerable range of drugs has been developed to tone down blood pressure excesses.

And my passion — diet. There is nothing to tell a casual observer whether eating a carrot rather than a chicken nugget is healthier or not. What we know (and it is now quite a bit) has come from researchers patiently assembling data about what people eat and seeing what happens to them. The central role of coloured plants, fibre, light processing, calorie restriction and oily fish are not obvious. They are only known to us because patterns were probed, and likewise, the deleterious roles of salt, alcohol and excessive fat.

There is now a red paver on Broad Street where the pump stood (the street is now Broadwick), and round the corner, a pub named the John Snow, which strikes me as wonderfully fitting. It is difficult to think of many doctors more deserving of glasses raised in their honour.

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