During her escape from the world of the Aen Elle, Ciri travelled through the spacetime continuum and one of her stops was likely medieval Europe in our World during the plague epidemic of the 14th century. She landed in a port town (perhaps Genoa or another Mediterranean port) and was immediately repelled by the place because of the masses of corpses, those of people and animals, lying about in the streets. There were also many fleas and many burned buildings marked by white crosses.
She immediately jumped to another point in the spacetime continuum, but before she was able to do so, a few fleas attached themselves to her jacket. Her next stop was that same port town where the Catriona was docked. She decided to quickly leave this place as well, but just before the next spacetime jump one of the surviving fleas fell from her jacket and made itself comfortable on an old sewer rat. This rat boarded the Catriona.
Later in the Saga, there was a small note about a dying tomcat who had caught a rat that left the hulk. This is how the so-called Catriona plague started.