In the Middle Ages, devil-fearing Christians killed cats, which carried the unintended consequence of increasing the rat population and the spread of the Black Death
In the 1300s, doctors had almost no knowledge about the causes of plague. As a result, there were countless superstitions about plague. Some Europeans blamed various groups as Jews, beggars, lepers, and friars, while others blamed animals. The best-known superstition was about the bad luck of cats and their association with the devil.
Many cats were killed as a result of this superstition, which exacerbated the problem of the plague. Without cats to act as their predators, little kept the rat population in check. People in Europe continued to kill cats for another 300 years and remained vulnerable to the plague when it swept through Europe again in the 1600s