Here's my take on the infection aspect of the show. Everyone's already infected. When they got to the CDC, the CDC Guy takes a blood sample from everyone and says, that it's just what he expected ... at the time, I took that to mean that no one was infected. Looking back, it seems like he knew that everyone was infected and he was looking for someone who had immunity ... which they didn't have. This is probably the secret that CDC Guy told Rick and what Rick was about to tell the guy (African-american guy with son) over the radio.
So, when you die (of causes other than trauma to the brain), you reanimate. If you get bitten by a zombie, the infection kills you, then you turn.
Not sure how they're gonna take it, but perhaps the baby is the key. Maybe the baby will end up having immunity/anti-bodies to the zombie disease. But at the rate that time passes per episode, this might have to wait until Season 16.