You know, one of the reasons I love zombies is they make the best metaphors of any movie monster so they make it easy to project insane political theories onto and so, last night while I was watching, I realized that zombies are completely intolerant bastards who want one thing and only one thing: They want you to be like they are. They're dead, they want everyone to be dead.
Which brings up the point that these zombies appear to really be dead, it isn't some stupid virus turning modifying living people's behavior.
For the most part, I like my zombies to be dead an rotting and animated for no apparent reason. There are a few exceptions, like Quarantine and the Resident Evil movies, but the RE movies star Milla Jovovich so they're going to be good no matter what. Anyway, my point is, I loved all the guessing in the early Romero movies about what might be causing the Zombie Apocalypse, whether it was radiation from a satellite falling back to earth or simply the fact that Hell is full, nobody is ever quite sure.
The most likely explanation for any true Zombie Apocalypse, of course, is Plan 9 from Outer Space. It's always Plan 9 from Outer Space.