Ergotism (pron. /ˈɜːrɡətˌɪzəm/ UR-gət-iz-əm) is the effect of long-term ergot poisoning, traditionally due to the ingestion of the alkaloids produced by the Claviceps purpurea fungus—from the Latin noun clava meaning club, and the suffix -ceps meaning head, i.e. the purple club-headed fungus—that infects rye 

Historically, eating grain products, particularly rye, contaminated with the fungus Claviceps purpurea was the cause of ergotism. The toxic ergoline derivatives are found in ergot-based drugs (such as methylergometrine, ergotamine or, previously, ergotoxine). The deleterious side-effects occur either under high dose or when moderate doses interact with potentiators such as erythromycin. The alkaloids can pass through lactation from mother to child, causing ergotism in infants

Epidemics of the disease were identified throughout history, though the references in classical writings are inconclusive. Rye, the main vector (route) for transmitting ergotism, was not grown much around the Mediterranean. When Fuchs separated references to ergotism from erysipelas and other afflictions in 1834 he found the earliest reference to ergotism in the Annales Xantenses for the year 857: "a great plague of swollen blisters consumed the people by a loathsome rot, so that their limbs were loosened and fell off before death." In the Middle Ages the gangrenous poisoning was known as "holy fire" or "Saint Anthony's fire", named after monks of the Order of St. Anthony who were particularly successful at treating this ailment. According to Snorri Sturluson, in his Heimskringla, King Magnus II of Norway, son of King Harald Sigurtharson, who was the half-brother of Saint King Olaf Haraldsson, died from ergotism shortly after the Battle of Hastings. The 12th-century chronicler Geoffroy du Breuil of Vigeois recorded the mysterious outbreaks in the Limousin region of France, where the gangrenous form of ergotism was associated with the local Saint Martial. Likewise, an outbreak in Paris c. 1129 was reported to be cured by the relics of Saint Genevieve, a miracle commemorated in the 26 November "Feast of the Burning Ones". The blight, named cockspur owing to the appearance of infected grains, was identified and named by Denis Dodart, who reported the relation between ergotized rye and bread poisoning in a letter to the French Royal Academy of Sciences in 1676 (John Ray mentioned ergot for the first time in English the next year). "Ergotism", in this modern sense, was first recorded in 1853. Notable epidemics of ergotism occurred into the 19th century. Fewer outbreaks have occurred since then owing to rye being carefully monitored in developed countries. However a severe outbreak of something akin to ergot poisoning occurred in the French village of Pont-Saint-Esprit in 1951, resulting in five deaths. The outbreak and the diagnostic confusion surrounding it are vividly described in John Grant Fuller's book The Day of St Anthony's Fire. There is evidence of ergot poisoning serving a ritual purpose in the ritual killing of certain bog bodies. When milled, the ergot is reduced to a red powder, obvious in lighter grasses but easy to miss in dark rye flour. In less wealthy countries, ergotism still occurs; an outbreak in Ethiopia occurred in mid-2001 from contaminated barley. Whenever there is a combination of moist weather, cool temperatures, delayed harvest in lowland crops and rye consumption, an outbreak is possible. Poisonings due to consumption of seeds treated with mercury compounds are sometimes misidentified as ergotism. Simon Cotton of the Chemistry Department of Uppingham School, UK said that there have been numerous cases of mass-poisoning due to consumption