What's in your head, in your head
zombie zombie zombie
Tricky question! Admund would certainly have heard about the undead in stories, but unless he's had personal experience he might not have believed in them. Also, stories about the undead mix up facts with fantasy (do vampires really have a compulsion to untie knots? Do they hate running water? Should you drive a stake through their heart and bury them face down at a crossroads, or cut off their head and burn them?).
I'll update later.