Claviceps purpurea is a gnarly purple fungus that grows on grain, specifically on wet grain. It looks like nothing so much as worms clinging to stalks of wheat.
If that grain is ground into bread and consumed, the bread causes ergot poisoning.
Ergot poisoning (or ergotism) has been known, if not understood, at least since the time of Lucretius, the first century B.C. There are two different forms of ergotism: convulsivus and gangraenosis.
Both forms of ergotism present with diarrhea, vomiting, mental impairment, and hallucinations. This is why sufferers of ergotism have historically been thought mad, or possessed. Ergotismus convulsivus is additionally characterized by profound visual, sensory, and mental disturbances, and contractions in the muscles of the hands or limbs, and paraplegia.
Patients with gangraenosis ergotismus suffer from lack of blood flow to the limbs, which often break out into blisters. The limbs become gangrenous, and either autoamputate or become septic. Hence the convent record which describes “a great plague of swollen blisters [that] consumed the people with a loathsome rot so their limbs were loosened and fell off before death.”
In 1090, when his son fell ill from ergotism, a French nobleman promised, before the tomb of St. Anthony, that if his son recovered, he would give his fortune to the church. St. Anthony, Anthony the Great or Anthony of the Desert, was an Egyptian hermit who lived in the third century A.D. He is often prayed to against skin diseases (perhaps not the most appetizing of saintly vocations).
‘The Temptation of St. Anthony’, Hieronymus Bosch, 1500-1525
When his son was well, that nobleman founded the Order of St. Anthony, monks who devoted themselves to caring for the victims of ergot poisoning. Over time, because of their efforts, ergotism became known as St. Anthony’s Fire.