I know some spoilers from the comic, but not too much. That being said, let's say everyone is infected.
Now my mind goes back to anyone who has died recently. For one, how about the guys in the bar fight. At minimum 2 died from gunshots alone in the bar, and the other was eaten by walkers. It did not seem like the 2 in the bar were springing back to life anytime soon.
That could be a timing issue, but it still leaves things unclear in my mind. Especially since Randall was in their camp, they were exposed to the same things, etc...
To me, it's all speculation for now. Who knows if the show will handle things in the same light. They certainly haven't been explicitly following the comics thus far.