Sometime around 14 July 1518, in the city of Strasbourg, a woman known to history as Frau Troffea walked into a narrow street and started to dance.

There was no music. There was no festival. She simply danced — and kept dancing, according to the chronicles, for days.

Within a week, dozens of other people had joined her. By August, contemporary accounts speak of hundreds. They danced in the streets and in the squares, sweating, exhausted, apparently unable to stop.

It sounds like a fairy tale. It isn't. The dancing plague of 1518 is recorded in physicians' notes, sermons, city council records and chronicles. Something really happened in Strasbourg that summer.

The question is what.

A city on the edge

Strasbourg in 1518 was a prosperous free city of the Holy Roman Empire. It was also a city under enormous strain.

The years before had brought crop failures, soaring bread prices and hunger. Smallpox, syphilis and leprosy were circulating. A mysterious and terrifying "sweating sickness" had swept parts of Europe. The poor were desperate, and the desperate were frightened.

And people in the region believed in something specific and terrible: that St Vitus, if angered, could curse sinners with a compulsion to dance.

That belief matters. Keep it in mind.

The cure was more dancing

At first, the city's physicians didn't blame demons or saints. They declared that the dancers were suffering from "hot blood" — a natural condition in the medicine of the day — and that the way to cure it was to let them dance it out.

So the authorities did something that sounds unbelievable now. They encouraged it.

Guildhalls were opened. A wooden stage was reportedly built. Musicians were hired to play drums, fifes and other instruments, and strong men were paid to keep the dancers upright when their legs gave way.

The idea was simple: dance until the fever breaks. Instead, the crowd of dancers grew.

It was the worst possible prescription. If the dancing was spreading through suggestion and spectacle, the city had just built it a stage.

The reversal

As the weeks passed and the dancers collapsed, the authorities changed their minds entirely.

The cause, they decided, wasn't hot blood but a holy punishment. Public music and dancing were banned. Gambling and prostitution were cracked down on as the kind of sin that might have provoked the saint. And the afflicted were loaded onto wagons and taken to a shrine dedicated to St Vitus in the hills above Saverne, where they were given small crosses and red shoes and led around the altar.

Over the following weeks, the epidemic faded.

How many died?

This is where the story gets slippery, and where a lot of retellings go wrong.

You will often read that "fifteen people a day" danced themselves to death. That figure comes from a chronicle written later, and it is not supported by the surviving records of the city council, which describe the epidemic in detail but don't record mass deaths.

It's plausible that some people died — of exhaustion, heart failure or stroke — after days of dancing in summer heat. But we simply don't know how many. Any precise number you see should be treated with suspicion.

So what caused it?

Three explanations come up again and again. They are not equally good.

Why it still matters

It's easy to laugh at the city fathers of 1518 hiring a band to cure a dancing epidemic. But consider how they were thinking: they had a theory, they applied it, it made things worse, and they changed course. That is not so different from how medicine still works.

And mass psychogenic illness didn't stay in the 16th century. It has been documented in schools, factories and towns around the world in modern times, from fainting outbreaks to a laughing epidemic in 1960s Tanganyika.

The summer of 1518 is remembered as a mystery. In a way, it's something stranger: a reminder that the line between mind and body, and between one mind and the next, is far thinner than we like to think.