*Horror-Nerd mode activated*
Zombies?
They're vampires.
VAMPIRES.
They're not the same thing. Yes, they're both dead people that still walk around and can spread their 'disease' with a bite, but Zombies usually don't have fangs, are rotting and their mental capacity usually doesn't go farther then 'Braaaaaains'.
Like, seriously, how can you mess that up?
Also, and spoilers for newer episodes here:
There will eventually be zombies. Zombie Pirates (Insert Pirates of the Caribean reference here) to be precise. So, uhm... how you going to explain that one?