Perhaps his most lasting action was a minor item: saying in his papal letter Vox in Rama of 1232 that cats were an instrument of the devil and a symbol of heresy. This led to a great reduction in the number of cats, which, a hundred years later, contributed to the quick spread of the Black Death plague, which killed 1/3 to 1/2 of the population of Europe. http://en.wikipedia.org/wiki/Pope_Gregory_IX
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Eight hundred twenty years ago
A little fact few people know
Caused history to change it's flow.
I swear it's so. I swear it's so.

Pope Gregory to all decreed
That cats were spawned from devil's seed
To kill them was a vital need
And with godspeed..and with godspeed.

Then, all throughout the countryside
A host of countries, far and wide
Conducted feline genocide.
The creatures died. The creatures died.

By thousands they were slaughtered by
Fanatics heeding Papal cry.
Light the bonfires! Watch them die
For God on high. For God on high.

Then came the dreaded plague to fall
On young and old. It killed them all.
The strong, the weak, the big the small,
See how they fall. See how they fall.

For many years, from dawn to dawn
Infected rodents traveled on
With nothing there to stop their spawn
The cats were gone. The cats were gone.