"I think that Creutzfeldt-Jakob disease would be the most likely source of zombie genesis, moreso than rabies or ebola."

"Creutzfeldt-Jakob disease (CJD) is a very rare and incurable degenerative neurological disorder (brain disease) that is ultimately fatal.
Symptoms:
rapidly progressive dementia
memory loss
speech impairment
jerky movements
changes in gait
rigid posture
personality changes (possibly violence)

The transmission is currently by things like eating mad-cow diseased beef, ski graphs/blood transfusion. But it isn't too far to think that it may evolve to be transmissable through something like, I dunno, a bite."

"One way for a virus to spread to a huge area very fast would be if it had a long incubation time and it was airborne, meaning that the infected people wouldn't notice anything for a long time. Maybe it could infect every human on the planet, but not actually do anything until they die, when it would somehow revive the person before the body and brain loses functionality.

"