Try not to be nervous.

https://owlcation.com/humanities/Cats-and-the-Black-Plague (full article)

Cats and the Black Plague

“. . . .From the middle of the 1300s until the 1700s, the black plague terrorized much of Europe and parts of Asia. Most historians believe the plague was first brought to Europe on ships from Asia. The most likely culprit was the black rats that often foraged among the ships’ holds for food scraps. These were smaller relatives of the brown rats.

The initial outbreak of the plague in fourteenth-century Europe was the most virulent. In fact, much of the populations of England and France were decimated. In some parts of England the death toll was 50%. Some parts of France suffered an astounding loss of ninety percent of their populations.

Many modern readers assume that there was only one outbreak of the black plague, but there were actually several. In fact, it raged through Europe about once every generation until the beginning of the eighteenth century. One of the last major outbreaks occurred in England with the Great Plague of London, which took place in 1665-1666.

Interestingly, the fate of mankind was curiously linked to that of the common house cat. When the cat populations rose, the pandemic ebbed, and when the cat population plummeted, the black plague made a resurgence. Why?

Remember that the plague was spread by fleas that lived on rats. A vicious cycle kept the disease going. Infected fleas would bite a rat, and the rodent would become infected. Then other fleas biting the infected rat would become infected themselves. Once the host rat died of the plague, any fleas living on it would find themselves homeless and would go in search of a new host. Unfortunately, this often took the form of a human. When the sick infected fleas bit the human in order to feed, the human would become infected. So why didn’t the Europeans just keep plenty of cats around to kill the rats and thereby reduce the incidence of the plague? They had cats at the time. They were originally brought to Europe by the Romans, who had discovered the felines in Egypt. Keeping pet cats as mousers had become popular in Europe by the time of the first plague.

To fully answer that question, you need to understand the belief system of medieval Europe. Based on historical accounts and medieval art, people during this period were prone to many superstitions. The Catholic Church was the most powerful entity in Europe at the time, and the masses were consumed with the presence of evil and eradicating it in any form it might be believed to take. Because of their secretive nature and their ability to survive extraordinary circumstances, the general population came to fear cats as consorts of Satan. The innocent cats began to be killed by the thousands.

The cats ultimately got their revenge, of course. Since there were few felines left, the rat populations increased unchecked, and the plague grew even more widespread. You’d think that the humans would make the connection by this point, but instead, they made things even worse. They began to associate the plague’s new vigor with the cats and even with dogs. They believed that since both of these animals typically harbored fleas, they must be the cause of the plague. Subsequently, cats were outlawed in many parts of Europe, and huge numbers of cats and dogs were killed. In fact, at one point in the middle ages, there were barely any cats left in England at all.

Even though cat ownership was illegal in some regions, a few people kept their felines. Other people finally noticed that these cat owners often seemed to be immune to the black plague. Word spread quickly, and more observations of this phenomenon were noticed. This resulted in research, crude as it was during the time.

Eventually, it was decided that the rats, not the cats, were responsible for spreading the black plague. Then, of course, everyone wanted to own a cat or two. And since cats are prolific breeders, it didn’t take long for the demand to be satisfied. The laws which had been the cats’ death sentence were repealed. In many regions, a new law took its place – one that protected felines instead of banning them and almost causing their extinction in Europe.”

I love all animals, but – I am a “crazy cat lady”, and proud of it. I have taken in strays, abandoned, and yes, feral cats, of all ages, from seniors to special needs, to newborn kittens, over the years. Many I kept as part of my own fur family, while others I socialized, with the purrfect help of my 4 amazing neutered male cats, and found loving forever homes for them. I have loved them all, and I cried every time one left, whether to cross over the Rainbow Bridge or to go to a wonderful family waiting to give them the love and care they deserved. I have kept in touch with them all, and although my thoughts had always been to better the lives of the cats, I didn’t realize what I was also doing for the families until I got this reply from one of my fur parents recently when I posted my thoughts and feelings over sending my last two fosters to their new home.

Jenny Haynes: “I know it’s hard being a foster fur-parent but in doing so you’ve brought sooooo much love to other people. If it wasn’t for you taking care of Herc despite his medical problems and coming back from other homes, we wouldn’t have the little family we have now. <3"

So yeah, it's worth the good fight.