Either the public has a terrible bout of necrophilia, or there's something about zombies that makes us come back for more. When I think of zombie fiction, I think of a chaotic world with little civilization and an abundance of shambling, flesh-eating freaks, a world in which your best friend is the timeless shotgun and the word of the day on Pee-Wee's Playhouse is "BRAINS!!!"
Zombies play on our fear of cannibalism. It's one thing to be eaten by a shark or a lion, but being eaten by your neighbor is another level of creepiness altogether separate. I think people have an innate fear of teeth, perhaps an evolutionary memory left over from when our ancestors were easily prey as well as predator. And unlike other predatory monsters - vampires, for example - there is no reasoning with a zombie. It's not some suave European stereotype that wants to seduce you. To a zombie, eating your girlfriend literally means eating your girlfriend.
The zombie apocalypse also makes for a great MacGuffin for a survival story. In order to have a good apocalyptic survival story, civilized order and society must break down. This can happen with any sort of disaster, and it's not restricted solely to the undead. Hurricane Katrina was a great real-world example of this with the looting and violence that took place in the aftermath.
Disease, zombie plague or not, can be even more alarming. When watching a documentary on the 1918 Flu Pandemic, one interviewed virologist said: "An epidemic erodes social cohesiveness because the source of your danger is your fellow human beings. The source of your danger is your wife, children, parents and so on. So if an epidemic goes on long enough and the bodies start to pile up and nobody can dig graves fast enough to put the people in them, then morality does start to break down." And, in fact, during the 1918 Pandemic, people were justifiably afraid that it was the end of human civilization.
Why is it that we don't see more pandemic fiction than zombie fiction? I think it's because zombie fiction can offers a slim chance of survival. When a disease like the flu is rampant, you can't hide from the germs. But if the disease is spread from bite-to-bite like rabies, you know that you can be spared by avoiding contact with the undead. Also, you can't see germs with the naked eye; zombies put a face onto the danger. It brings about a clear "us versus them" mentality. Even among the survivors, trust can be a rare thing because everyone wants to live and sometimes that means putting others in danger.
With zombie fiction, we also get a chance to start over. Stories such as Justin Cronin's The Passage feature colonies of survivors banding together. Whereas we now live in a world of big cities, civilization retreats to the small town. In a small town, your relationship with your neighbors is more intimate because they're your main source of human contact. That's not to say that it's all picture perfect, but the interpersonal contact is much more noticeable. I once heard about a poll conducted in Los Angeles - the most populous city in the most populous state of the U.S. - and the prevalence of loneliness was surprisingly high even though you're surrounded by millions of people.
That, I think, is the secret to the success of zombie fiction: a simultaneous fear of great masses of people alongside an attraction to small groups of people. Going back to that poll on loneliness, perhaps zombie fiction is an exaggerated reflection of real life. We're all surrounded by complete strangers, any of whom can be a danger, but we're seeking out individual survivors in the horde.