Slow the infected zombies to the speed of a player in standing double time. But add infected hounds that run as fast as the current zombies, or faster. You can already use the animations and models of the in-game dogs (mongrel) possibly, add these wild dogs into the woods or in small towns. Dogs could be attracted to player corpses or the corpses of dead animals (or both) adding this would make hunting, Pking, and just running in the woods more risky.