I'm sure you know this already Jim. but you can use undead or demons or inanimates, they don't get affected by disease.
And in a game I played with Cleveland, somehow he managed to sneak Wardens into every single province I owned without too many of them getting diseased -- talk to him!
C'tis PD is teh suck, they get a pile of weak morale 7 militia (8 in C'tis dominion) that die easily, causing the not-as-terrible City Guard to rout. You can Send Lesser Horror, Arouse Hunger, Horde from Hell, etc, maybe even Call of Winds, to take his PD-only provinces.
Depends on the nation you are playing, but usually there's an answer.