Check the Civ-Content folder, there's already a withplague.biq (don't remember the exact name).
In general, maximum plague (like in the Middle Ages Conquest) is brutal, but in the epic game, its effects are pretty random. For most of the time, it won't hurt you much, but I had a game where I had to defend against a (stronger) neighbor, and exactly the crucial border town was hit by plague, killing off lots of defenders...
And don't forget it almost for sure will hit your best cities, since it is trade-related...