rumor around TWD subreddit is that when Dr Jenner whispered in Ricks ear, that he said everyones already "infected".
i havnt read the comics, so i dont know the validity of the statement, and i dont spoil myself, so its just a guess
I'm starting to think this as well. Everyone's infected, it just doesn't "activate" (for lack of a better word) until the person dies.
I was glad yet surprised that Shane died, I thought Rick was going to convince him to come back and they'd act like the most hostile best buds and then a couple episodes later it'd be back to it again but they pleasantly surprised me that they killed him off. I applaud you writers