I think it's like the comic book. Everyone is already infected and unless you have head trauma when you die, you come back as a zombie.
I don't think those were zombie flashbacks when Shane died, I think that was whatever zombie-causing-micro-critters running through his synapses to turn him into a walker, and they wanted the audience to share in the fun.
I think that we find out what the CDC dude told Rick next episode. Probably what we already came up with, that everyone is already infected. I also wouldn't be surprised if we don't see another death next episode. Sarah Wayne Callis (plays Rick's wife) has said that it isn't a matter of *if* she dies, but *when*. The only one safe on this show is Rick.
And I love the idea of a baby zombie eating Rick's wife from the inside out. I'm pretty twisted too.