hmmm, I just noticed that you never do see who is responsible for the infection or even the reason.
In the original, there is some kind of space radiation that brings the dead back to life. In the 80's Return of the living Dead, it is a chemical called Trioxin.
Getting bit does not make you a Zombie. It infects you with... Something. That Something causes you to die. Once you die, you return to "Life" as a flesh eating Zombie.
LOL!
Yeah, I'm a big Zombie movie geek.
I do not believe in the dead rising to attack the living. Not in any way, shape form or fashion. I do believe, however, in the possibility of a chemical or biological weapon that ,once released, could cause large numbers of people to utterly lose their minds. Ala, 28 days later or The Crazies.