Did you know that extermination of cats may have been responsible for the Black Death? The plague that swept Europe in the 1300s and killed as much as half the human population there, might have been at least partly averted if more cats had been around. David Grimm, author of "Citizen Canine: Our Evolving Relationship with Cats and Dogs," discovered that little tidbit when conducting his research for that very book.
It's no secret that cats help to keep rodent populations in check. That's the primary reason they were once so popular on farms, and what led to their domestication in the first place. In ancient China, and ancient Mesopotamia, cats were drawn to human habitations because of the large supply of prey. They tolerated us, and we tolerated them because they provided natural pest control. In time, we domesticated them.
In the year 1232, Pope Gregory IX instilled in the people a fear of cats, saying that Lucifer, the devil, sometimes appeared as a cat, and was actually half-cat. Religious zealots in Europe sought to kill cats, then, seeing them as incarnations and associates of the Devil.
Such widespread extermination would have led to a massive growth in the rodent population, because they no longer had predators in abundance. It was the fleas on rats that spread the Black Death, and with no cats around to kill the rats, both them and their fleas flourished, and their populations grew unchecked and out of control.
Of course, that isn't proof that cats' extermination at that time directly led to the Black Death. However, it is a logical conclusion to draw.