If it isn't the governor or one of his lackeys, my bet is on the black bald dude (the one that Daryl brought home like a pet in one of his jaunts) or the two new little girls.
Aside from them, I don't know any other characters. Tyrese's sister can't possibly be the culprit since she's going to get sick as well in the next episode. It's going to be really stupid if the twist is that it's someone from within the group that they haven't introduced before.
I'm also surprised that Daryl is the new leader. But we all know Rick's still in charge if he wants to. Daryl isn't the kind of guy who'd want to hold on to authority anyway, unlike Shane or the Gov.
