Mass hysteria is when a bunch of people go crazy at once. One way mass hysteria can develop is if a whole lot of irrational people get together at some religious event, then they all get worked up and emotional together.
Nuns forced to live repressed and frustrated lives could be at risk of developing mass hysteria, below is an example:-
A nun in a German nunnery fell to biting all her companions. In the course of a short time all the nuns of this convent began biting each other. The news of this infection among the nuns soon spread, and it now passed from convent to convent throughout a great part of Germany, principally Saxony and Brandenburg. It afterwards visited the nunneries of Holland, and at last the nuns had the biting mania even as far as Rome. (Zimmermann cited in Hecker 1844, 127) 
 There must have been a great many unhappy, frustrated nuns in the convents of Germany, Holland and Italy.
Suggestibiblty can have terrible effects especially among credulous people who are already unhappy and frustrated.