In "The Walking Dead", somewhere at the end of Season 2, Shane turned into a walker without being bit. Later Rick tells the group what Dr. Jenner had earlier confessed to Rick in private, that they "all had it in them", and by default all the dead would turn into walkers.
This contradicts the scene in the first episode, where hundreds of dead were lined up at the entrance of the hospital, where Rick was admitted when he was shot. Why do they not turn?
So is this a mistake? Or is there a timeline event where all got infected after a certain period.
I have not read the graphic novels, and have completed season 3.