The plague which occurred in Good Sister, Bad Sister and was referred to in each of the other books of the epic series The Heart of the Staff was caused by Ugleeuh. Ugleeuh had designs on Prince Hebraun of Niarg, who was betrothed to Princess Branwen of Far. She fancied herself such a raging beauty that Hebraun would surely fall for her in spite of her having no peerage, if only she could eliminate Branwen. And she went to a great deal of trouble to do so. She followed the instructions that she found in a grimoire which had long been hidden for everyone's safety.
She had to find a young girl infected with Yersinia enterocolitica, with a high fever and bloody diarrhea and pains in her right lower belly and in her joints. It took her some time because the girl also had to have a pet kitten with lice. She put a spell on herself for protection, took the kitten and put it into a basket with a tight lid lined with linen and said a long incantation which caused a gene substitution that changed the Yersinia enterocolitica to Yersinia pestis, the dreaded plague. She then arranged for a courier to deliver the kitten to Branwen as a present from Hebraun. Branwen fell ill at once, releasing the pestilence into the countryside.
Ugleeuh's Father Razzmorten (who knew none of this) learnt of a cure called worrobobo from Ngerrk-ga, an Ngop shaman on the Dark Continent, and set about making it, stopping the plague.