Mass hysteria, for the purposes of this article, refers to the manias in which a group of people all begin to exhibit shared physical or psychological symptoms, for which there is no direct cause. The earliest recorded are the dancing manias which swept Mythic Europe. Mass hysteria is poorly understood, but documented cases have several factors which align, with a striking congruence, with the life of companions in Ars Magica settings.
Mass hysteria is far more common in group who live separated from the rest of society. Most covenants are physically separate, because of the dimming effect of magic of large numbers of believers in the Divine. There is also a social distancing in being covenfolk: many have fled feudal society, and live outside the understood confines of the Three Estates.
All incidents of mas hysteria occur in communities under stress. That stress can be economic, but also includes social change and disasters. Covenant life is meant to be a respite from adventuring for magi: but it isn’t a respite from danger. There’s a reason most magi live in castles. Mythic Europe is a dangerous place and sometimes the thing that wants to kill you really is on the other side of a stone wall a yard thick.
Incidents often occur in people who have little social standing. There’s some debate here, as to if the conditions usually appear in people of rank, and then flow into the lower strata of society. That being noted, companions live in a feudal society. The magi are at the top: they are at the bottom.
Outbreaks of mass hysteria occur in populations which skew female. I know how sexist that sounds, but the data is in on that one, I’m afraid. In small covenants that keep to traditional gender roles, when the magi leave with their shield grogs, the remaining population skews female.
Mass hysteria, in slightly later than the game period, was often seen as a form of spiritual possession, caused by faeries or demons. There are also other alternatives. An attack by another covenant could take the form of contagious madness. A very early Ars Magica supplement had a covenant plagued by the dreams of a dragon, about to hatch from its egg, deep below the covenant. The hysteria may be a result of a Twilight experience suffered by a resident magus, who appears unaffected, but acts as a carrier. It may be caused by poisoning, due to the residue of magical experiments getting into the covenant’s water supply.It may signify that the covenant’s Aura is uneven, and people working in one particular area have been Warped to an unexpected degree. This may be the precursor for eruption of a previously hidden regio.
The Vessels of Iniquity are a already designed as expressions of contagious hysteria. A spare one I wrote up for Antagonists is Terpsichore.