[Paine in the Neck]

Ergot? Fungus on the rye that was a northern European staple of the time. Produces hallucinogenic effects. Suspected to be a contributing factor in the witch burning hysteria of the time.

[corkoman]

Moribund the Burgermeister - Peter Gabriel from his first solo album.

[Zirondelle]

Sounds like a town in England - Fungus-on-the-Rye.

[Rockingham]

My guess as well. Agriculture and storage methods of that era were conducive to the development of fungus on grain. Immune function and fertility were thereby suppressed, along with high infant mortality and intermittent outbreaks of ergot poisoning. Notably, the famine associated with the Strassbourg dancing mania would have led to the consumption of even rye grain visibly tainted by ergot fungus, which in better times was usually discarded.

Since the ergot toxin normally survives both baking and brewing, it could have easily found its way into the products of such processes. Flour and beer variably tainted by ergot could explain the episodic and inconsistent pattern of the Strassbourg dancing mania. A few tainted barrels of beer or batches of flour could easily cause an outbreak in a city even while most people were unaffected.

[SunkenCiv]

Apparently ergot was suggested over the years, but the symptoms don't match up (it sez here). Ergot has also been blamed for the Salem Witch Trials -- that seems to me to have been due to the fact that human beings, for all our good qualities, can be giant a-holes.