Sounds pretty cool. But now that I've started thinking about Civ IV with zombies, I can't think of anything else.
Just imagine if there was a "bioweapon" technology. You could research it, then launch it as weapons at your enemies and watch it go so far out of control it becomes a threat to all civs, including your own. And if you're particularly unlucky, it may get accidentally unleashed on your own people.
Now imagine it's not exclusive to zombies and has randomly determined characteristics. Maybe it's airborne, maybe it feeds on radiation. No way to know how to stop it until you get the appropriate technologies to examine it. But, uh oh, maybe an accident while examining it....