Zone Yellow — Keith Laumer

"Brion Bayard, once of our own timeline and now Imperium Agent extraordinaire, had been on some pretty dangerous missions before -- but never had he encountered so noxious a foe as the invading legions of giant plague-ridden rats who walked like men, spreading disease across the multiple universes of the Imperium. Unless Bayard can travel to the original world of the long-tailed invaders and stop the plague at its source, the Earth of the Imperium and all the other Earths in all the universes will fall before the verminous hordes from a timeline that should never have existed in the first place..."