I tried before - I had a rough outline of a young married couple (with a 9-month-old daughter) who were living in the midst of a flu-like pandemic that ended up being a zombie pandemic. I had it all worked out from beginning to end, but I just wasn't comfortable enough with the story to write it. I admit, it was a bit weak, the characters were kind of derivative, the ending was sort of a cop-out and there weren't enough zombie encounters, plus I just couldn't bear the thought of putting that imaginary baby through that kind of world (my daughter was the same age at the time i came up with the story, so that probably has a lot to do with it). Anyway, I gave up on it before I even finished writing the outline.
But then The Walking Dead aired and I got caught up in the zombie craze and decided to start again. This time I had a better, more fleshed out (hehe) story, complete with multiple zombie encounters. I had a couple of flawed main characters (which is important - if your characters are too perfect they're not believable and therefore your audience won't connect to them), a daring escape from a zombie-infested hospital, a slightly less harrowing rescue from a zombie-infested apartment building, all wrapped up in a budding romance, with the death of a much-loved family member and conflicts with other surviving humans to boot.
I got pretty far in writing it, and I really did like this story (and still do), but I got to a point where I couldn't go any further. Not because I was having trouble figuring out what was going to happen next, but because of one, big, overriding factor:
Zombies really really creep me out; more so than any other supernatural creature.
Ghost and demons are scary, sure, but in comparison to the other supernatural baddies they're pretty easy to deal with - talk to them sternly enough or say the right words and they go away. Alien invasion is so far removed from the realm of possibility (even though they're the one thing from this list that might actually exist somewhere out there) that it doesn't even bear considering. Witches and gods, fairies and magic, psychics and telepathy - they're just cool, and to be or to have any of those things would be absolutely fantastic. Werewolves and vampires are cool, too, and can be very sexy as well. Plus, depending on the mythology you follow, a vampire/werewolf bite can be cured (wolfsbane, killing the sire, refraining from drinking human blood). Even if you can't be cured of either affliction you can still live a (somewhat) 'normal' life. If you retain your sentience while in wolf-form, you can be pretty baddass, and if not, well then just get someone to lock you in a cage for 1-3 days a month. Same with vampires - sure you're essentially dead, but hey, you're immortal, super-strong, and sometimes you have telepathic or shape-shifting abilities. So what if you have to avoid sunlight and churches and italian food.
But zombies, they're not cool or sexy by any means. Zombies are disease anthropomorphised. They are big, scary walking viruses with fingers to claw at you and teeth to gnaw at you (and eat your braaaaaiiinnnns) and have a virus' purpose - consume and spread. They're completely mindless so you can't reason with them or win them over to fight for the good side. They're not magical or even technically supernatural beings so you can't send them away with a cross or an incantation or a bible passage. And there's no cure - once you're bitten you're done for, and you might as well shoot yourself in the head right then and there (well, unless you've got some Zombie Tarts). All it takes is the smallest drop of infected blood getting in your eye, in a cut, in your mouth, and it doesn't matter if you beat off the zombie that was attacking you, you're still gonna become one of them.
You don't get any cool powers, either - all you have to look forward to is a lot of shuffling and moaning and your body parts falling off one by one. And even though walking, reanimated corpses with an insatiable hunger for human flesh are basically impossible, horrible diseases and fast spreading pandemics are not, and add to that the breakdown of society and resulting apocalypse and OMG how the heck are we going to survive without running water and grocery stores and central heating in the wintertime and that's what makes zombies so disturbing to me.
Yes, I know it's all make-believe, and if I'm watching a zombie movie or t.v. show it's not such a big deal because when the movie/show ends, the zombies go away. But I live with the stories I write for a long time. If I'm writing a story about a bunch of people trying to survive a zombie apocalypse, I have think about how I would survive it, and outline it in detail so I can describe what the characters are doing and why they're doing it. The story is in my head morning, noon and night, and the main character is pretty much always another version of me, so I often feel like I'm the one faced with surviving a zombie apocalypse.
I've since started another zombie story, although this one is still inside my head (and may stay there, who knows). Third time's the charm, it seems, and I like this one even better than the last. Oddly enough it was inspired by the new (not zombie invasion but alien invasion) series Falling Skies (okay I admit it, I was inspired by Noah Wyle in Falling Skies) and it's made me realise that what really draws me back to the zombie apocalypse story is not the zombies themselves, but the struggle to survive in a post apocalyptic world. How would we survive if we really had to fend for ourselves, if we didn't have access to electricity or doctors or the internet? How would we survive a Canadian winter without central heating or the ability to go grocery shopping? That's what I find fascinating, and a 'zombie apocalypse' is just an excuse to explore that aspect of the human experience.