A social phenomenon that occurred primarily in mainland Europe between the 14th and 18th centuries involved groups of people, sometimes thousands at a time, who danced uncontrollably and bizarrely, seemingly possessed by the devil. Men, women, and children would dance through the streets of towns or cities, sometimes foaming at the mouth until they collapsed from fatigue. [...]Although no real consensus exists as to what caused the mania, some cases may have had an explicable physical cause. The symptoms of the sufferers can be attributed to ergot poisoning, or ergotism, known in the Middle Ages as "St. Anthony's Fire". It is caused by eating rye infected with Claviceps purpurea, a small fungus that contains toxic and psychoactive chemicals (alkaloids), including lysergic acid and ergotamine (used in modern times as a precursor in the synthesis of LSD). Symptoms of ergot poisoning include nervous spasms, psychotic delusions, spontaneous abortion, convulsions and gangrene; some dancers claimed to have experienced visions of a religious nature.
The question in my mind is how/why a fungus would evolve to make people dance en masse.
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