In 1233, a priest by the name of Konrad von Marburg received reports that a group of heretics were operating in Germany. Being a fairly proactive holy man, Konrad launched an inquisition to root out these supposed heretics, because if there was one thing God didn't like, it was being worshiped in slightly the wrong fashion. This being the thirteenth century, launching an inquisition of course meant terrorizing random people, threatening to burn them at the stake, and torturing the shit out of them until they confessed to pretty much whatever Konrad wanted to hear. Amazingly enough, Konrad discovered that not only were there heretics in Germany, but a full blown satanic cult. According to Konrad's "investigation" members of the cult would kiss the asshole of a black cat before engaging in a wild homosexual orgy which upon culmination would result in the arrival of the devil, who would then apparently just hang out for a bit of satanic chit chat. Flush with holy vigor, Konrad sent a letter describing his findings to the Pope in Rome. However, he was soon after mysteriously murdered for reasons that can only be described as obvious given his habit of torturing random innocent people.
When Pope Gregory IX got Konrad's letter he freaked the fuck out, which may have been somewhat understandable if anything in the letter was true. While other priests also wrote letters, most describing how Konrad was a torture happy asshat, the Pope decided to go with Konrad's by far much more interesting description of what was going on. To be fair to Gregory, Germany was a long ways away, and as anybody who has ever seen German pornography can tell you, Germans are kind of weird people when it comes to such shit. Anyways, thoroughly freaked out at the thought of a bunch of feline butthole kissing orgy loving cultists, Gregroy wrote a Papal edict commanding the nobles of Germany to hunt down and eradicate all signs of the supposed cult.
Gregory's edict left the German nobles in a bit of a quandary. On the one hand, they wanted to prove they were good Christians by totally doing what the Pope told them to do, but on the other, the satanic cult they were supposed to eradicate totally didn't exist. Luckily the nobles found a loophole. The edict did mention that black cats were used in the cult's evil rituals, so if the nobles just wiped out all of the black cats, then the cult wouldn't be able to do the aforementioned rituals. No black cats meant no buttholes to kiss which meant no cult. What followed was exactly as ridiculous and stupid as you would expect.
Imagine you are a serf living in the Middle Ages. Your entire life is pretty much toiling your ass off with no hope of bettering your position until you drop dead before your fortieth birthday. You're illiterate, as are all of your friends, and you probably have livestock living in your house with you, if you're lucky enough to own livestock. Now imagine that your local priest, the most educated person in town, gets up on the pulpit and commands you to kill all the black cats you see because they are totally the devil or something like that. Now imagine how easily things get out of hand today via social media even though we’re all supposedly well educated. So yeah, it’s not hard to see how the whole killing black cats thing quickly got out of control. The idea that black cats were the devil soon spread across most of Europe, and then morphed from just black cats to pretty much all cats. Over the next 110 years the people of Europe pretty much killed any cat they could get their hands on.
Funny thing about cats. They eat rats. Funny thing about killing all the cats. You end up with a lot of rats. Throughout the Middle Ages, Italian trading vessels regularly plied the waters between Europe and the Middle East. Starting in the mid-fourteenth century, they began bringing back not just delicious spices and the such, but also rats infested with fleas which in turn carried the Bubonic Plague. Yeah, you can probably see where this is going. Over the next decade, the rats, and hence the plague, spread across Europe, killing an estimated 30 to 60 percent of the continent's population, some 75 to 200 million people. Sufficed to say, things did not go well. Of course, the people of Europe, seeing the error of their ways, stopped killing cats soon after. Wait, that's not right. Actually they doubled down on the whole cat killing thing, resulting in several more rounds of plague over the next several centuries. In fact, they didn't stop needlessly killing cats until the early nineteenth century, and to this day black cats are still considered bad luck.
Image: https://commons.wikimedia.org/wiki/File:Black_cat_IMG_1618.jpg