Cats were brought to Europe from Egypt by the Romans and enjoyed a decent reputation for a long time—probably because they were such a boon to agricultural societies. Vermin did a number on harvests, but cats were nature’s perfect solution: they literally can’t eat anything that isn’t meat, which means that they pose no risk to the crops.

But feline-human relations deteriorated sometime in the early 1230s (CE) when Pope Gregory IX issued a papal bull called Vox in Rama. This bull, the story goes, declared cats as the instruments of Satan, and set Medieval Europe on a great cat purge, with special attention paid to black cats, who were particularly Luciferian.

From there, it’s just a couple of logical leaps to the connection to the Black Death: inspired by Pope Greg IX’s bull, everyone across Europe killed off all the cats. Then, when the rats came bearing the Plague, there were no mousers left to keep the disease in check.

More Info: museumhack.com