As historical facts suggest, Pope Gregory IX hated cats. The Pope associated cats with devil worship and loathed these felines so much that he ordered extermination of cats throughout Europe. 

With decreasing number of cats, it was easier for the rat population to flourish. This, many believe, helped spread the Bubonic Plague, also known as Black death. 

The Black Death was the deadliest pandemic recorded in human history. It resulted in the deaths of up to 75–200 million people in Eurasia and North Africa, peaking in Europe from 1347 to 1351.