Even though the CDC doctor told Rick that everyone is affected, that may only be his theory. Shane did turn into a zombie after death with no visual markings of an infected bite or scratch that we saw on the episodes to support that assumption.
The writers of the show may have a surprise in store for the mid season finale or the end of the season finale.
I'm still betting on the shaved head woman who likes Darryl to be the next major person to die. The season opener gave you the impression it might be someone else but 2nd episode hasn't aired yet.
I have been in awe waiting for this season to start, I don't think Darryl will die - he's to much of a survivor, I would point to Glenn - his risky behavior so far and love for maggie that "Crippled" him once will get him killed. Not sure exactly what Carol
has to do really with the story, after season 2 the entire plot was around her daughter - now she's dead, It seems she's either going to be Darryl's lover or end up dead.
If I had my pick I'd say kill Carol off.