That's still a possible theory, but with Shane dead now there is no way they can come to that realization since he was alone on the bus. Besides, it doesn't explain Randall's infection.
It's becoming pretty clear that everyone is a potential walker with death being the cause.
@DDustiNN, the guy in season 1 got sick because he was only injured(scratched? I don't recall), he hadn't died yet. The wound was obviously infected though, so he would have died and turned eventually. As for the two guys in the bar, I believe Rick finished them with shots to the head, so they didn't have a chance to turn.