In 1518 in Strasbourg, France, a woman known to history only as Frau Troffea began dancing in the streets. Initially she drew excited crowds who cheered her on, but as the minutes turned into hours, Frau Troffea continued to dance and dance and dance without pause.

Hours became days and still Frau Troffea continued to dance with no breaks, as though she had no control over her own body and was unable to stop.

The situation worsened as other Strasbourg residents began to join her. By the end of the week, 34 people were dancing uncontrollably. By the end of the month, there were 400 dancers.

The endless exercise took its toll and at the peak of the phenomenon fifteen people per day were reportedly dying of strokes, heart attacks, and sheer exhaustion.

After about a month, the epidemic came to an end as abruptly as it began and those who survived ended their dancing and returned to their lives.

What became of Frau Troffea—whether she survived or died as a result of her unending dancing—is unknown.

This strange event remains well known 500 years later and has come to be called the Dancing Plague. But is this event unique? Far from it.

Many examples of so-called "dancing mania" have were reported between the 7th and the 17th centuries, some extensive and damaging, others little more than small curiosities.

In the 1020s in Bernburg, Germany, eighteen peasants began dancing uncontrollably in church, disrupting a Christmas Eve service. In 1278, 200 people danced on a bridge over the river Meuse, ultimately leading to its collapse.

Between 1373 and 1374, outbreaks of dancing mania were reported in England, Germany, and the Netherlands.

A case of dancing mania in 1237 involved a group of children and has been suggested to be the real-life origin of the story of the Pied Piper of Hamelin, which originated around the same time.

Throughout the 17th century there were further reports of dancing mania, but they now tended to be single patients rather than a contagious epidemic, and since then the phenomenon has vanished entirely.

So, what was the cause of this bizarre behaviour?

There are suggestions that some kind of epilepsy or seizure is the cause, but it's worth remembering that the mania is always described as dancing: the movements are never suggested to be random. Uncontrollable, yes, but co-ordinated enough to be recognisable as a dance.

This makes chorea much more likely than something like a seizure. Chorea is a movement disorder that causes large movements, particularly of the limbs, that are smooth and have an illusion of being intentional, although they are in actuality completely involuntary.

Chorea is often a symptom of the genetic Huntington's Disease, but it can also be acquired as a consequence of HIV or streptococcal infection.

To witness chorea when you're unfamiliar with it can create a bit of an uncanny feeling—the distinctive movements don't look the way you would expect involuntary movements to look. They often resemble gestures that a person might make intentionally, and can certainly resemble rhythmic dancing.

This explanation would require, however, all the sufferers to acquire the same infection and have the same reaction. It also fails to explain why the outbreaks often resolve themselves so quickly.

So, are they other theories on what causes dancing mania?

In Italy, dancing mania has also been known as tarantism and was thought to be the result of a bite from the lycosa tarantula spider—a separate species to the one we more commonly call "tarantula."

This particular spider bite was said to cause the uncontrollable dancing we see in dancing manias. It was even thought that the dancing saved the victim, that it somehow counteracted the effects of the spider bite and kept the dancer alive.

This belief even spawned a style of dance we still know today—the tarantella.

This belief even spawned a style of dance we still know today—the tarantella.

Ergot is a type of fungus that grows on rye and can cause serious health effects in humans who eat grain contaminated with ergot.

Effects include headaches, nausea and vomiting, diarrhoea, spasms, and psychological effects like mania and psychosis. It also causes vasoconstriction of blood vessels and the long-term descrease in bloodflow can cause parts of the body to become gangrenous.

Ergot also produces a chemical known as ergotamine. It's commonly used as a medical treatment thanks to the vasoconstricting properties which make it useful for combatting bleeding and migraines.

Ergotamine, however, is also used in the production of LSD, which has led some to wonder if hallucinogenic symptoms might also occur from prolonged ergot poisoning.

It does seem possible, from the health effects brought on by ergotamine and its parent fungus, that this theory could adequately explain the strange behaviour exhibited during the dancing plague. But whether the people involved had even consumed ergot is unknown, and so it's not something we could every confirm one way or the other.

A final theory turns inward and looks at human psychology for an explanation of dancing mania.

Mass psychogenic illnesses are illnesses that, while they may present with wide and varied physical symptoms, have psychological or neurological origins with no clear physical cause for the physical symptoms.

At its most basic, this means physical symptoms present despite the illness existing only in the patient's head. Which is not to say they're making it up: these illnesses are very real, and the symptoms uncontrollable and distressing. People with these illnesses have no conscious control over what is happening to them.

And worst of all, these illnesses can be contagious.

In 1962, a village in modern-day Tanzania experince mass psychogenic illness first hand when three girls at a boarding school began to laugh uncontrollably. The laughter quickly spread, eventually affecting 95 children, all of which experience uncontrollable episodes of laughter that lasted from a few hours, to more than two weeks.

The laughter eventually spread outside of the school and into neighbouring villages, eventually affecting 14 schools and a thousand people. The epidemic eventually fizzled out after 18 months.

Over in Britain, a similar issue affected a school in Blackburn, Lancashire, where 85 girls were taken to hospital as a wave of dizziness and fainting spread through the students.

There were many proposed explanations, such as viruses, food poisining, or a gas leak, but no concrete cause was ever found and the incident has since been put down to mass psychogenic illness.

The problem with this theory is that it can be hard to imagine how such a thing can be possible: how can people exhibit such bizarre behaviour with no conscious control, just because those around them are doing it?

A highly publicized case of mass psychogenic illness in Malaysia in 2018 has allowed us a closer look through first hand accounts.

"I was at my desk feeling sleepy when I felt a hard, sharp tap on my shoulder," explained Siti Nurannisaa, the 17-year-old who sparked the outbreak. "I turned round to see who it was and the room went dark."

Siti continues to describe a sudden and terrifying psychological break in which she hallucinated dark and grotesque visions of demons and "a face of pure evil."

Siti began to scream and soon the terror spread to the other girls in the classroom. The screaming became an uncontrollable panic and grew and grew. By the end of the day, 39 people were affected by what was dubbed "mass hysteria."

Could this contagious hysteria be a modern example of a dancing plague? Examples of dancing mania slowed down in the 17th century and are unheard of today, but is this simply a result of changing cultures? That that which once presented as dancing now presents as laughter or as screams? (Or meowing?)

While history leaves us too distant from 1518 to ever solve this mystery for sure, the past hundred years presents us with countless examples of contagious psychogenic illness that might provide us with a greater understanding of the Dancing Plague of Strasbourg.