Hello and welcome to more or less on the BBC World Service.

We're the programme all about the numbers all around us in the news and in life, and I'm Tim Halford.

During this pandemic, data has undoubtedly been one of the most powerful tools we've had at our disposal.

This week, we're stepping away from our current crisis and taking a journey way back to the birth of epidemiology in the mid-1800s.

The person who'll be taking us on this journey is Stephen Johnson, who's the author of various books, including the Ghost Map, about a remarkable piece of data detective work.

So it's 1866 and kind of an anomalous outbreak develops in the east end of London near the River Lee, which was one of the most polluted rivers in the entire country in the kind of industrial working class east end of London.

This was a cholera outbreak and it was anomalous because at that point cholera had been on the decline in London for some years. Now it was back.

And initially just two people died, a couple, Mr. Mrs. Hedges, who lived in Bromley by Bow.

And then in the weeks of followed, Londoners started to experience something that's all too familiar in the times of COVID-19, which is reports in the newspapers of a mounting death toll.

First it was 20 people and then the next week it was 40 people and then it was 80 people.

It was widely accepted by this point that cholera was a waterborne disease.

That consensus had been reached as a result of the work of physician John Snow a decade earlier.

Snow had noticed a spatial pattern in the data.

And Snow basically analyzed patterns of death in a Soho neighborhood in London in an outbreak that took place in the late summer of 1854 and created a famous map that showed that the deaths were concentrated around a pump where people were getting contaminated drinking water, which caused people to get sick.

And that investigation is what first really convinced the medical establishment, the scientific establishment that cholera was in the water and not in the air.

Because of this work on cholera, John Snow is now considered one of the founding fathers of epidemiology.

But by the time of the 1866 outbreak, Snow had passed away.

The mantle was taken up by a man called William Far.

Far had come from a poor rural family. He was an early member of the Statistical Society of London, later the Royal Statistical Society.

And this passion for statistics also took him to a post in the General Register Office, a new government body tasked with tracking births and deaths in England and Wales.

So when this outbreak began, he's sitting there at the General Register Office overlooking all the data on illness in the greater metropolitan area and in fact throughout all of England.

And he sees this pattern emerging in the East End. And so he decides, OK, immediately this must be a problem of a contaminated water supply.

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