I did that with the Scipii's African provinces during my particular Roman Civil War. In my experience the plague totally buggers a city's income level, so having a bunch of infected spies traipsing around enemy territory is a neat way to hurt their wallet.

And when the old spies go and die of the plague replacing them is a simple matter of infiltrating a fresh, healthy young spy into an infected settlement...

...
...you know, for some reason I think I should feel at least a little bad about such nasty tactics...

"Son, Rome needs your help, your Sacrifice !"
"Yessir, what am I to do ?"
"Go and catch the plague and spread it around. Now off with you, and don't write home 'cause I sure don't want to get sick."
"...yessir..."