[Optimus Prime]

The thing of it is that he immediately reanimated as a Walker. It usually takes a few hours, with Shane being a bit quick because, according to the writers, he was in a murderous rage when he died and so there was more "life" in that corpse. Nobody has ever keeled over dead and reanimated literally two seconds later, though. He died, hit the ground, and then a moment later his eyes were open again. So I think whatever killed him has to be related to the Zombie Virus, since it not only killed him but reanimated him instantaneously.

[RandallFlagg]

The thing of it is that he immediately reanimated as a Walker.

I don't know if it really was immediately. After he cools off in the shower (Presumably because the zombie virus victim develops a massive fever), he drops. Then, the scene of others sleeping in their cells cuts in, then it cuts back to him lying on the floor with a bloodied face. A good amount of time might have passed between shots.

[Hot Tabasco]

The thing of it is that he immediately reanimated as a Walker.

We don't really know that, all we saw was the kid claiming to be sick in Carol's class, excusing himself, then walking thru the prison coughing, then going into the prison to take a shower. It's there where he collapses and apparently dies. We really don't know the time frame between the death and when he reanimates............

As a side note, we have seen many instances where a killed person reanimates very quickly.....Shane being a prime example.