St. Gertrude of Nivelles is the unofficial saint "cat-lady" for her intercession against mice during the plague.
While St. Gertrude of Nivelles is known as the patron saint of cats, this title was given to her not because she loved cats, but on account of her ability to expel mice.
St. Gertrude was a Benedictine nun during the 7th century and was known for her deep piety. She founded a monastery in Belgium and died at 31 years old.
There exist no traceable stories that connect her to cats or even mice during her life, but during the Middle Ages she became a favorite saint to invoke against mice and rats.
The Catholic Encyclopedia explains, “She is also patron of travelers, and is invoked against fever, rats, and mice, particularly field-mice.”
Furthermore, in an early 20th century art-journal it explains that she is often depicted with mice around her.
We see her represented on a fine tapestry at S. Mary’s Hall, Coventry, dressed as a nun, with a mouse on each shoulder and one floating or flying above her head. Champions of medieval piety protest that these figures are allegoric. But it is certain that until a few years ago, even if the practice has been dropped now, the guardians of the Church of S. Gertrude at Nivelles sold water which was guaranteed to preserve a building from the invasion of mice.
With this in mind, in recent years St. Gertrude became associated with cats, since cats are notorious for their ability to keep the population of mice and rats down in an area. It is now common to see religious images and icons of St. Gertrude with cats, making her the unofficial “cat-lady” of the Catholic Church.