The Means of the Zombie Virus

In response to Verloren's comments about the source of the zombie virus...

Meteors
This is the primary vehicle for the zombie virus in Night of the Living Dead. It would explain the spikes in the zombie menace throughout history. However, I have my doubts (yet, nothing definitive) about this method, since the extreme heat upon entry into the atmosphere would likely kill anything living.

Mutated Virus
The similarity to the Rabies virus (transmission through bites, the virus attacks the brain causing changes in behavior, etc) makes this a possibility. Of course, Rabies has yet to reanimate corpses.

Latent Human Virus
A variation on the mutated virus; this is a virus that has been around for a good portion of human existence, but for whatever reason has remained unactivated. Something (whether it be genetic or a mutation, or some combination) has allowed the virus to be activated through outside circumstances. However, I find this also to be unlikely because this would mean that certain individuals would have a resistance to the zombie virus. As of yet, no resistance has been documented.

These are just a few suggestions on an area that I have not researched as well as some other areas (such as weaponry). I welcome any additional theories.