Okay, Okay, I can respect the wordsmithing, let's see if I can extrapolate and get on board.
First I think, we have to turn OFF the assumption that "the dead have risen" to become zombies... the creatures we see aren't dead people turned, they are LIVE people who got infected and turned... that takes care of the "whatever killed them" element. They were never dead in the first place.
So an infected person , aka "zombie", gains his hunger for flesh, gets tougher fingernails, yada yada, then goes to eat a live human. That live human gets eaten and dies, and we never see that person again (as they do not "turn" and become living again). Those victims NEVER become "zombies".
BUT, if a zombie passes their infection to another, that victim DOES show the wounds from their initial attack that infected them, and does turn into a zombie, and is therefor still susceptible to damages, like piercing, blunt or slashing, because well, they're still alive.
Yeh I can get on board with this.