During her escape from the world of Aen Elle, Ciri travels through the time-space continuum and one of her stops is probably medieval Europe in our World during the plague epidemic in the 14th century. She enters a port town which could be Genoa or some other Mediterranean port. She is immediately repelled by the place because there are the corpses of people and animals lying in the streets. There are also many burned buildings marked by white crosses and many fleas.
She escapes to another point in the time-space continuum, but before she is able to do so, a few fleas stick to her jacket. Her next stop is another unnamed port town. She is able to discern that the port lays in Nilfgaard. She decides to quickly leave this place as well, but just before the next time-space jump one of the surviving fleas falls from her jacket and makes itself comfortable on an old sewer-rat. This rat boards an old dirty hulk which sets sail that very evening for Cintra. The name of the hulk was Catriona. Later in the Saga, there is small note about a dying tomcat who had caught a rat which left that hulk.
St. Lebioda's Hospital in the Temple Quarter of Vizima is filled with the victims of Catriona, and Shani spends her time trying to help them in Chapters II and III. She probably works in the hospital during Chapter IV, as well, but Geralt is no longer in Vizima to witness this.
According to the old women in the Outskirts, when the plague first came, the first to go were the cats and dogs, then the rats and finally the people. Apparently, the lack of food even drove some of the more desperate to cannibalistic and necrophagic acts and many ghouls were created as a result.
The plague is rampant across the North's war-torn countryside in the wake of the invasion by Nilfgaard's legions. Keira Metz will find a cure to the Catriona plague in the years that follow the end of The Witcher 3 if she is invited to Kaer Morhen.