Article:
D&D 4th ed: Zombie Outbreak

Zombies are standard fare for a Dungeons and Dragons game, but not a plague of them. When faced with an actual, deadly zombie contagion, how will your average D&D player, and their character, react?
As a one off “What if” setting, or perhaps a deadly twist that you may decide to throw into a campaign (kind of a D&D iron man challenge) the Zombie Contagion can be simple, or it can involve a bit of complexity, to make it more of a mystery.
Now I should mention that I have, of late, been an avid reader of The Zombie Hunters, and this article features the same basic model for a zombie disease that the creators of that web comic series have made.
So, the general idea for a zombie disease is that it is seriously bad news to get infected, and to be infected means that as soon as you die, the disease will cause your corpse to rise and become a zombie.
The specifics of the disease are that you must be bitten by a zombie or have ingested zombie flesh (or fluids) in order to become infected. The infection itself is a dormant stage of the disease, but it can still be transmitted through the blood (not very likely in the D&D world), and there are few cures outside of divine intervention or injecting holy water directly into the blood stream (which might cure you, or kill you).
While living with the disease, there are no outward signs of infection, you might have no idea you are infected unless examined by someone trained in healing skill (DC 20 to detect the disease).
However, as soon as you are next reduced to below zero hit points (yes, even just -1), you will become a zombie.
As a player character, this means irreversible death.. ceasing to be a player character under your control and becoming just another monster under the dungeon master's control.

As we all know, zombies can be killed quickly by removing the head or destroying the brain, so in the brief time that the player character is dead and not yet a zombie, other players can decapitate them.. who knows, maybe they can be restored at that point using resurrection magic.. I leave that up to your DM.