About Left 4 Dead

bleeding - border case as this only occurs when a player gets incapacitated and waits for someone to assist them.
hypermobile foes - hunters mainly, the others have only slightly higher mobility than the protagonists.
karmic creatures - boomer, killing him in close quarters has the same effect as having him barf on you.
killing blow - the common infected are so very fragile. not applicable to the special infected.
elite npcs - the special infected.

asymmetric factions - versus mode, the infected and the survivors.