^ That's certainly possible. I wish the writers would have shed some light on what the CDC guy really did say to Rick.
In the bar fight a few episodes back, did Rick shoot both of the bad guys in the head or elsewhere? I need to go back and re-watch that episode. If he shot them in the chest and called it a day, there's a good chance that Rick doesn't know that everyone's infected.
He shot the skinny guy in the head and the fat man twice in the chest and once in the head.
Of course though that is just how police are trained to shoot assailants.