Very good question.
Not a lot of folks made it to the CDC to have their blood analyzed.
The immunity does explain why Rick took his sweet time lining those three walkers up that were on top of him to try to get’em all with one shot.
I was also wondering about those punks from Randall’s camp in the bar.. did Rick brainpan both of them? I can’t remember. If he didn’t, they should have turned.
I can't remember, which episode was that?
I just watched last weeks episode. What I gathered is that back in the CDC center, the scientist whispered to Rick that everyone was infected. When people get bit they get infections that cause them to die. Then the “walker” infection takes over when they die.