I'm having an argument with friends over whether a zombie story requires zombies.

Their point of view is that if there are no zombies then it's not a zombie story, period.
My point of view is, a zombie story is about the characters, not the zombies. It's about a setting where:

- there's no time or resources to do anything but simply struggle to survive.

- You're outnumbered and outgunned and can't even sleep unless you've barricaded yourself in six ways from Sunday and set reliable guards.

- The fallen return as your enemies, no matter how close you were before.

- You live in constant fear that, should you fall as well, you'll become part of the enemy completely against your will, the ultimate indignity. "You" might not even exist anymore, just your soulless body operating against your will.

- You know that if you slip up, you face the likelihood of watching yourself "turn" and helpless to do anything about it. Honestly, I think this fear is probably the strongest theme in the "zombie" story that sets it apart from other horror... getting mobbed by zombies, meh, that's just a brutal death like any slasher/gore piece. The part that really gets the viewer/reader is watching the character allllllmost get away, then a simple bite... but not so simple.

- Catastrophic collapse of social order is often a key element; you can't trust the people around you not to trip you as a distraction so they can get away, steal your supplies, etc... fragile new forms of social order grow to replace existing ones, often just as dangerous as the zombie threat. And if you get bitten, or someone accuses you of it for an ulterior purpose, your place in any social order immediately disappears; you instantly become vulnerable to the whims of those around you.

- Character development revolves around whether you remain human by protecting others, or become a monster yourself by sacrificing other survivors for your own safety

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I feel like when you take those critical themes in mind, you realize that you can replace the zombies with pretty much anything and still tell the exact same story.

Vampires decided to live in the open, hunting and preying on humans. Survivors band together for safety, but you can never be sure if one has secretly turned.

Alien parasites, a cult that forces demonic possession on those they capture, a psychic that can use mind control; in a high tech setting it could be a data virus that infects your neural interface, that's spread through physical contact to avoid detection on the 'net...

What do you folks think? Is it possible to tell a zombie story without zombies, or are zombies themselves a key element that just cannot be replaced?