St. Anthony is closely connected to the fire, in two very different senses. On one hand, the bonfire is meant, as large bonfires in winter have a symbolic purifying and healing function, on the other hand, the disease called "St. Anthony’s Fire" is to be mentioned.
Anyway, St. Anthony mastered the fire and not surprisingly the iconography represents him often with the flame in the palm of his hand.
A popular legend tells that Anthony went to hell with his faithful pig to redeem the souls of some dead. The piglet by running around created havoc among the demons, while Anthony recovered the damned souls. With his rod he caught some fire from the hell and brought it on the earth to heat and give light to humans.
Image: the Focara Bonfire at Novoli, Italy. Copyright Antonio Zaccaria, source: http://www.fondazionefocara.com/
St. Anthony’s fires are huge bonfires lit on the hills or in village squares to celebrate the light and the victory of the saint over the devils. The begging (described in the previous paragraph) also served to collect the firewood, the "wood of St. Anthony."
In Italy, in the Marche region, there is the custom of the "faori", or fires of joy, lit the night before the feast; in Abruzzo the “focaracci” or “focaroni” are widespread, which are huge flaming torches lit on the crossroads or in the squares; in Puglia instead of wood people burn reeds called “farchie”.
The fire was considered healing: so in Abruzzo at Collelongo people lit two oak torches, towering over 5 meters, which burned all night long and were visible from the great distance. Each villager then brought home a piece of ember as a protection against thunders, storms or devils.
There was also the custom to let a fire wheel tumble down a hill or run through the fields. The aim was to spread the light symbolically in the dark winter, to heat and to purify the earth.
St. Anthony’s Fire was then classified medically as corresponding to the two different diseases: ergotism, caused by a fungus, and herpes zoster, caused by varicella-zoster virus. The latter is creating painful rashes in the shape of a belt (zoster means "belt"), but Ergotism was more spread in the Middle Ages. Ergotism is caused by a fungus, Ergot (Claviceps purpurea), a parasite of cereals and especially of rye, which, when affected by the fungus, gets a kind of cockspur, pending from the grasses. If this rye was not carefully cleaned but instead milled and used to produce a flour contaminated by this fungus, it caused serious intoxication to the people. The consequences were severe heartburns, spasms, convulsions, seizures, hallucinations, colic, vomiting, gangrene, mummification of fingers and toes and consequently the death.
Image: painting of a patient suffering from advanced ergotism from approximately 1512–16 AD. Detail from the Isenheimer Altar by Matthias Grünewald (c. 1470 – 1528), a German Renaissance painter of religious works.
Source: Matthias Grünewald [Public domain or Public domain], via Wikimedia Commons
The chronicler Sigiberto Genbloux wrote: "Human flesh fell to pieces, as men were burned by a sacred fire which devoured and gradually turned their body black as coal. They quickly died in agony or continued their life without the feet and hands, a life worse than death...".
These cases, spread mainly in rural and poor areas, were attributed to the work of the devil. The fire brought by the saint was seen as a purifying element, while the fat of the pig was used to heal the sick, more precisely to isolate wounds and maintain the infection limited over the body; hence the vocation of the Antonian Order to the treatment of the Ergotism.