In the first half of season 2 of Buffy the Vampire Slayer, Drusilla is quite weak. She is unable to hunt on her own, often bedridden, and Spike is constantly worrying over her. Her illness is apparently so severe that a magical ritual involving her sire is required to cure her. Do we know what caused this illness?
There is some mention of a mob in Prague that attacks Spike and Dru but I don't recall ever hearing what exactly they did to Dru. Vampires are able to recover from physical damage, so I would assume whatever happened to Dru had a mystical component to it.
I would prefer an answer from the Buffy or Angel tv series or from the Dark Horse Comics continuation but if an answer cannot be found there, I will accept answers that come from other extended works.