Ergot, Claviceps fungi,Claviceps purpurea,paspali, sometimes called cock spurs, and the effects of ergot poisoning from infected rye and the life cycle of this fungus was first written down in the early 1800s. In humans the delivery system was bread made from the infected grain. Last case I know of was 1950s in eastern Europe.
Ergot flourishes in damp conditions as does rye so the two tend to be linked but most cereals can be suseptable. Ergotism aka St Anthonys Fire/Holy Fire has also recently been linked to the various witch hunting and burning episodes in Europe and the u.s.
Ergot restricts small blood vessels and damages nerves causing a burning sensation of the extremities, hence the fire bit of its old name. Also it contains a chemical very similar to lysergic acid dithalimide lsd and can cause hallucinations.
Monks of St Anthony the Great specialised in treating the Fire so they may've had an idea where it came from.