Medieval Europe is often considered to have been in the “Dark Ages,” a rat-ridden era swamped with plagues, chaos, and mindless violence, but this isn’t strictly true  – they also had deadly dancing plagues. 

In mainland Europe between the 11th to the 17th century, there were a number of bizarre reports of the so-called dancing plague, also known as St. John's Dance and St. Vitus' Dance (Vitus being the Catholic patron saint of dancers).

So the stories go, groups of people would start erratically dancing in a state of frenzied delirium. The groups would grow with more entranced dancers and musicians joining in, sometimes gaining thousands of participants until the scene looked like a nightmarish landscape painting by Hieronymus Bosch. In some of the most severe “outbreaks”, dozens of people would reportedly dance until they died of exhaustion. 

One of the best documented “dancing plagues” occurred around June 1374 in Aachen in modern-day Germany, then part of the Holy Roman Empire. One account explains: “They formed circles hand in hand, and appearing to have lost all control over their senses, continued dancing, regardless of the bystanders, for hours together, in wild delirium, until at length they fell to the ground in a state of exhaustion.”

“Where the disease was completely developed, the attack commenced with epileptic convulsions. Those affected fell to the ground senseless, panting and labouring for breath. They foamed at the mouth, and suddenly springing up began their dance amidst strange contortions,” it reads.

Some even suggest that the 14th-century mania jumped from Aachen to nearby cities around the Holy Roman Empire found in modern-day Germany, Netherlands, and Belgium, as told by dozens of sources across Europe.

In more recent years, several theories have emerged in an attempt to explain this curious phenomenon. A 2009 paper in the medical journal The Lancet took a deep dive into the “forgotten plague” of dancing mania and explored a range of possible explanations. 

A popular theory is that the people were poisoned by flour contaminated with ergot, a fungus that grows on rye. Long-term exposure to the alkaloids produced by the fungus can cause a range of symptoms including convulsions, spasms, mania, and psychosis. Ergot is known to contain lysergic acid, which is one of the chemical building blocks found in lysergic acid diethylamide (better known as LSD). A few other similar diseases or contaminants have been suggested, but it appears unlikely that any acute epidemic would cause a festival-like outburst.

Another possibility is a mass trance-like state caused by extreme psychological distress from decades of poor harvests and vicious diseases. Alternatively, some have noted that the phenomenon bears similarities to possession rituals seen in a wide variety of cultures around the world.

Perhaps one of the most reliable explanations is that this was a form of mass psychogenic illness, sometimes called mass hysteria. This describes an event in which groups experience similar psychological or physical symptoms – such as fainting – in response to a threat, whether real or otherwise. While exceptionally rare, there are a number of significant cases of this phenomenon in the modern world. In 2011, around 18 people at a New York high school suddenly developed uncontrollable twitching, spasms, and verbal tics, much like the symptoms of Tourette's syndrome. There was also a fainting epidemic that affected some 1,000 people, primarily, teenage Palestinian girls near the West Bank in 1983. Just like medieval dancing plagues, modern-day mass psychogenic illnesses can be hard to distinguish from the effects of actual exposure to environmental hazards.

Writing the Lancet study above, historian John Waller concludes that the phenomenon of dancing plague is most likely explained through a pick and mix of these theories. He contends that the masses were likely shaken by decades of intense psychological hardship and fell spellbound under a widely propagated cultural idea, namely the fear of a dancing curse by St Vitus and St John. As he concludes, "a psychic epidemic had been turned into an ecstatic religious ritual."