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We like to think virality is a modern invention, powered by code and connectivity. But in 1518, one city proved that human behaviour has always been contagious – no technology required.

In 2026, it doesn’t take much to make millions of people do the same thing at once. A dance on TikTok, a phrase repeated often enough on X, a format that spreads so quickly it begins to feel less like a choice and more like a reflex. We tend to think of this as a feature of the internet age, something born out of algorithms and attention spans and the peculiar economics of social media. We call it “going viral”.

Five hundred years ago, in a small European city, something else went viral – but without a screen in sight.

A woman steps into the street

In the summer of 1518, a woman in Strasbourg (in modern-day France) left her house and began to dance.

There was no music playing that anyone else could hear, but the woman danced anyway. She did not stop after a few minutes, or even a few hours. She ignored concerned relatives and friends who tried to talk to her. She continued through the day and into the night, returning to the street again the next morning as if compelled by something she could neither explain nor resist.

At first, she drew a crowd. Then, gradually, she drew participants.

Within a week, several dozen people had joined her in the dance. By the end of the month, the number had swelled into the hundreds. They danced in the same streets, in the same heat, for days on end. Contemporary accounts describe blistered and bleeding feet, bodies collapsing from exhaustion, and a growing sense of alarm among those watching from the edges. Some sources suggest that deaths followed, though the exact number remains uncertain.

The city had not planned for this. There was no precedent to guide them and no framework through which to understand what they were seeing. What they had instead was a rapidly spreading case of choreomania, aka dancing fever. 

A pattern of contagion

Strasbourg’s outbreak earned its place in the history books because it is one of the most well-documented examples, but it was far from unique. Across medieval Europe, there are repeated accounts of similar episodes of choreomania, each one as baffling as the last.

In 1374, a wave of dancing mania swept through Aachen in Germany and spread outward, reaching towns across the region with a speed that would have been impressive even by modern standards. Chroniclers described crowds moving in unison, sometimes for days, sometimes until they could no longer stand. Earlier still, in 1237, a group of children in Erfurt (also in Germany) reportedly danced their way to Arnstadt, a journey of just over 20 kilometres that left observers struggling to make sense of what they had witnessed. It’s rumoured that this event inspired the writing of the Pied Piper of Hamlet. 

Medieval records tell of disrupted church services, processions derailed by sudden outbreaks of movement, and communities caught in cycles of behaviour that seemed to spread from person to person without any clear cause.

What makes these accounts striking is not just their frequency, but their similarity. Different places, different decades, and yet the same underlying pattern: an individual begins, others follow, and within a short span of time, the behaviour takes on a life of its own.

Diagnosing the undiagnosable

When the Strasbourg outbreak gathered momentum, the city’s leaders turned to medicine, or what was called medicine in the early 16th century.

This was a discipline still shaped by humoral theory, where imbalances in bodily fluids were thought to explain most ailments. Physicians examined the dancers and arrived at a diagnosis that, to them, seemed entirely reasonable. The condition was attributed to “overheated blood,” a state in which excessive heat in the body needed to be released.

It was, by their standards, a logical conclusion. The summer had been unusually warm, and the idea that physical agitation might be linked to internal heat fit within the medical thinking of the time. The prescription followed naturally from the diagnosis: if the problem was heat, then the solution was movement. The afflicted needed to continue dancing until the excess had been expelled.

City officials accepted this reasoning and acted on it immediately. Rather than attempting to contain the outbreak, Strasbourg’s authorities chose to accommodate it.

A stage was erected in the city centre, near the horse market, to give the dancers a designated space. Musicians were brought in to provide accompaniment, their rhythms intended to guide and perhaps even regulate the movement. Strong men were hired to assist those who faltered, lifting them up so that they could continue to dance.

The intention was curative, but the outcome was the opposite. By formalising the behaviour, the city transformed a spontaneous outbreak into a sustained event. The presence of music gave the dancing a structure it had previously lacked, while the stage turned it into a spectacle that drew further attention. What had begun as a disturbance became, in effect, a festival.

The number of participants did not decline. If anything, it grew.

Observers who might have kept their distance were now drawn closer, and those on the margins found themselves stepping into the centre. The boundary between witness and participant blurred, and with it, any sense of control the authorities might have hoped to maintain.

A change in belief, not in behaviour

As the days passed and the situation failed to improve, confidence in the original diagnosis began to erode. The city’s leaders were forced to reconsider their approach, and in doing so, they turned to a different framework of understanding.

If the body could not explain what was happening, perhaps the answer lay elsewhere.

The thinking shifted from medicine to morality. The dancing was no longer seen as a physical ailment but as a manifestation of divine displeasure. It may sound like a leap in logic to our modern ears, but in a society where religious belief shaped nearly every aspect of life, this interpretation carried weight.

The response changed accordingly. Music and dancing were banned outright, and the emphasis moved toward penitence and ritual. The afflicted were taken to a shrine dedicated to Saint Vitus, a figure believed to wield the power to inflict or relieve curses. There, they were made to participate in ceremonies intended to restore balance. Accounts describe the use of red shoes, the performance of specific rites, and the hope that submission to a higher authority might succeed where earthly measures had failed.

Throughout this shift, one detail remained constant: the dancers continued.

The limits of explanation

The dancing plague of Strasbourg lasted for almost two months. At its peak, an estimated 400 people were dancing at once. While exact figures are debated, some historical accounts suggest that up to 15 people per day were dying from exhaustion, heart attacks, or strokes.

The eventual end of the outbreak is recorded with far less detail than its beginning. At some point, the numbers dwindled, the movement slowed, and the city returned to something resembling normal life. What caused the cessation is no clearer than what caused the onset.

Over the centuries, various theories have been proposed. One of the most persistent is ergot poisoning, caused by rye infected with the fungus Claviceps purpurea. In damp conditions, the fungus replaces the grain with dark growths called sclerotia, packed with toxic alkaloids chemically related to lysergic acid – the precursor to LSD. The result can be vivid hallucinations, muscle spasms, and a creeping disorientation.

In medieval Europe, where rye was a staple, exposure wasn’t unusual. Entire communities could consume contaminated grain, triggering outbreaks of what was called “St. Anthony’s Fire”, marked by convulsions, seizures, and distorted perceptions of reality.

At first glance, it feels like a neat explanation for irrational, collective behaviour. But the details don’t quite hold. Ergot poisoning tends to incapacitate rather than mobilise. Severe cases leave people too weak or disoriented for prolonged activity, and symptoms are typically episodic – not the sustained, synchronised dancing seen in Strasbourg. As a standalone explanation, it struggles to account for the scale and duration of the outbreak.

The theory that carries more weight today is mass psychogenic illness – collective behaviour shaped by shared stress and belief. Strasbourg in 1518 was already a city under strain, worn down by famine and disease. In that kind of environment, the line between individual experience and collective response starts to blur.

The dancing, in this view, was not random. It may well have been a manifestation of a society under pressure, expressed through the only language it had available.

What remains

There is a temptation to treat the dancing plague as an isolated curiosity, a story that belongs firmly to the past. Its details are strange enough to encourage distance, and its explanations are rooted in a worldview that feels far removed from our own.

And yet, the underlying dynamics feel oddly familiar, even in our modern age. 

A behaviour emerges without warning. It spreads through observation and imitation. Attempts to control it serve, in some cases, to accelerate it. Explanations shift as understanding falters, moving from the physical to the moral to the symbolic. Eventually, the phenomenon exhausts itself, leaving behind nothing but a record.

In 1518, this played out in the streets of Strasbourg, with bodies in perpetual motion and a city struggling to keep pace with what it could not explain.

In 2026, the setting is different, the tools more sophisticated, and the scale far larger. The platforms may have changed, but the choreography feels familiar. Somewhere, someone starts something. Others follow. Before long, participation begins to feel less like a decision and more like a current that sweeps you away.

About the author: Dominique Olivier

Dominique Olivier is the founder of human.writer, where she uses her love of storytelling and ideation to help brands solve problems.

Her first book, Lessons from Loss, has been published by Penguin Random House.

She is a weekly columnist in Ghost Mail and collaborates with The Finance Ghost on Ghost Mail Weekender, a Sunday publication designed to help you be more interesting.

Dominique can be reached on LinkedIn here.