Ergot is a fungus than can explain the occurances of the famous 'plagues of witchcraft' that occurred in Salem, Mass. in 1892. Apparently, their rye crops were infected with a poisonous fungus that, when ingested, caused hallucinations (of all the senses), uncontrollable convulsions, and eventually death. There has been a similar case in France in the 1950s, where a baker unwittingly used a batch of flour that was made from rye infected with ergot. About 200 people in the small French town were affected, and (if I remember correctly) 17 people died. There was even video footage taken of the fungus' effects, which showed EXACTLY those symptoms described in the Salem journals that described the incident.

It was also found that ergot infections of staple crops could explain 'witch hunts' dating back to the middle ages. Weather conditions at the time were found to be conductive of ergot infection throughout areas where witch hangings took place. These areas were found to be near primary rye-growing areas of the time. Peasants were most affected, as their poor diets consisted mostly of food derived from that grain.

More interestingly, such behaviour of 'witch accusations' traces back to a petrified man found in a peat bog whose age has been estimated at approximately 2000 years. The man's stomach contents were found to contain large amounts of ergot, which was found to have been absorbed into his bloodstream. The man suffered a clearly brutal death, as his skull was cracked and his throat cut ear to ear. The discovery of ergot in his stomach would explain his brutal death, as the mystics and druids at the time would have thought him 'possessed' by demons and hunted him down.

Ergot is same fungus from which D-lysergic acid diethylamide (LSD) was originally derived. There are numerous other chemical constituents of the fungus (notably ergotamine) that cause convulsions of smooth muscle (like that in arteries) primarily in the cranial region, which would account for the uncontrollable convulsions as the brain was starved of oxygen.

It's also interesting to note that people called in a bishop to exorcise the site of the bakery where the poisoned bread made from the infected rye originated in that town in France.