Originally Posted by BigRock
He didn't do that to Shane, though. He was a sitting duck with Zombie Shane coming up behind him.
If that's what the CDC guy told Rick, it doesn't make sense that he'd keep it a secret. Not telling the others would just be putting everyone in danger.
Someone could die when Rick's not there. Choking, an accident. Hershel's daughter almost killed herself. And if nobody's expecting the dead person to turn, they're screwed. Hershel has a heart attack and dies, Maggie's bawling and goes into the bathroom to wash her face. She comes out and he jumps off the bed and eats her. Thanks, Rick.
I think he was debating on doing it to Shane and wrestling with the fact that he'd just killed his best friend when Carl showed up and distracted him. Rick has no clue how quickly the dead rise, so I don't think he thought he was in immediate danger.
I agree about telling the others, though. If the guy said, "We're all infected," it might have taken Rick a while to figure out the implications of that statement, and he might have just been working off of a suspicion when headshotting everyone. He didn't have definitive proof, and he probably didn't want to worry the group needlessly until he figured it out for sure (which is very true to his character).