In medieval Europe, Saint Anthony was the patron saint of those stricken with ergotism, an affliction with symptoms including burning pain and gangrene in the extremities, hallucinations, psychosis and even death.

In William Ellis' The Country Housewife's Family Companion, published in 1750, he states that St. Anthony's Fire "generally proceeds from excessive heat in the blood occasioned after surfeits, or by too free a use of spiritous or other strong liquors, and commonly causes great pain in the part it comes out in."

Although the link wasn't well-known until later, ergotism is in fact caused by eating bread made from rye infected with Claviceps purpurea, a fungus that produces ergot -- a mix of toxic alkaloids. Damp grain storage conditions caused the fungus to flourish, and often entire villages would be stricken. Mass ergotism didn't begin to appear until the Middle Ages, probably because rye wasn't widely cultivated for food until that time.

Among the mix of alkaloids in ergot are neurotoxins and peripheral vasoconstrictors. Vasoconstriction in the extremities was the cause of the burning pains and gangrene; some cases were so severe that fingers and toes or even hands and feet were lost. This quality of ergot alkaloids was exploited as early as the Middle Ages, when midwives would use a small quantity to speed up labor and reduce bleeding. In modern times, derivatives of ergot such as ergotamine were extracted and synthesized, and used to stop hemorrhage and relieve migraine. The work of Sandoz chemists, including Albert Hoffman and others, with ergot derivatives led to the synthesis of LSD (most famously), but also of dihydroergotamine (a migraine medication), Methergine (used for postpartum bleeding), Hydergine (developed as a treatment for dementia), and Sansert (another migraine drug).

The most recent outbreak of St. Anthony's Fire occurred in 1951 in Pont-St. Esprit, in Provence, France. Patients began turning up in the hospital complaining of abdominal pain, pain in extremities and cold fingertips. Babbling and hallucinations ensued. After two days, 70 homes were used as emergency wards, and straitjackets had to be called in from surrounding areas because the patients that escaped would soon be running frantically through the streets. All in all, 200 people were made ill and 4 died. As it turns out, all those stricken had eaten bread from the same bakery, which had obtained tainted grain from an unethical farmer.

http://www.botany.hawaii.edu/faculty/wong/BOT135/LECT12.HTM
http://pubs.acs.org/hotartcl/mdd/99/marapr/forged.html