[B0rsuk]

Now that you mention it, I think undead should be affected by miasma. They should decay pretty quick. Isn't that what Maggots spell is about ? Undead are, except for ghosts etc, organic matter. Some of them stale and dry, but also unprotected by human skin, immunity system and so forth.
But disease is another matter. Perhaps somewhere in future patch (or mod) undead will become disease carriers, able to spread disease without being affected ? Yuck.