STOP CALLING THEM ZOMBIES THEY WAS not ZOMBIES!!
Zombies come back from the dead and eat people
They didn't really die the virus took over their body and was in control of it this was prove later in the film when the chick questioned it and it happened to her and she turnt and was able to force herself to speak while getting control to say "kill me"
So it was pretty much like the movie "the faculty"