From what I've learned about creatures of myth, legend, and folklore zombies have essentially remained unchanged throughout history, a human corpse that had been brought back to life. However the purpose behind their reanimation and how it is accomplished has changed. In the original tales from Haitian culture a person of great magical powers, like a witch or necromancer, could use their powers to bring a person who has died back to life. The corpse would then be used as a slave with the bonus of being tireless and having no need for food. Essentially the zombie was a source of labor.
What then has changed? Most of the early media stuck with the original legends of zombies as servants created through dark magic to serve the forces of evil. The answer: Night of the Living Dead.
Ever since the stories that feature zombies have changed. No more are the undead servants of powerful witches and warlocks, but are now predators that eat anything and anyone they can find. It also no longer matters how old or rotten the bodies are. If the person is dead, they are guaranteed to come back to life.
What exactly causes the person to come back from the dead? Well there have been many causes used throughout the years. Like in Romero's film exposure to radiation has the power to get the dead back on their feet. One common factor used more recently is the idea that a virus of unknown origin could cause a person to die, and then come back with a taste for the living. I've seen one TV show and read one book where the cause of the zombie revival was the use of parasites which infest a person, kill them, and then take control of the functions of the body. Whatever the cause, the effect is still the same. The majority of the population gets wiped out and being bitten by a zombie causes you to become a zombie, much like in the legends of the vampire.
George A. Romero may not deserve all the credit. In the Epic of Gilgamesh there was a moment where the goddess Ishtar vowed that there would come a time where the gates of the Underworld would open and the dead would eat the living. As far as I know there hasn't been a film, game, book or TV show that has used the threat of Ishtar as the cause for the zombie apocalypse, but it would be interesting to see that used.
Before anyone asks, no I do not plan to use zombies or any related type of undead creature. I'm not a fan of horror and there's a lot I still don't get about zombies. First off, if they are supposed to be reanimated bodies, why do they only have a taste for that which is living? Shouldn't they still have the same body functions as when they were alive? Last time I checked we don't have a basic instinct that tells us to go around eating everyone we know, especially raw. Second, I still don't get why people insist that the undead problem is irreversible. If it is caused by a disease, it should be curable. Granted there are diseases that cures aren't known for. That doesn't mean they don't exist. We just haven't found them yet. Third, I don't see how a body that is deteriorating and rotting away would still be able to function so well. Surely the muscles would not be able to last and endure the stress so well. The organs must surely be tearing themselves apart all the time.
My last reason deals more with the matters of religion. I do not mean to offend anyone here. These are my personal thoughts. I believe very strongly in the Resurrection, and when I think about it I see bodies becoming zombies as a mockery of the idea that the dead would return to life. It makes it seem like we should fear seeing those who leave us come back from the dead because they'd be interested in eating our brains (and the rest of us for that matter). Also to me the zombie apocalypse idea always makes me depressed and sad because it makes it seem like there would be no hope for humanity. All we would've accomplished would be lost. Every day would be lived in fear, scavenging around for enough food and worrying that you could get attacked by something that should be dead. I do not mean to offend anyone who enjoys stories of zombies. If you like them that's your personal choice. I was just sharing my opinion of why zombie stories are generally not my favorite.