Here's something I didn't know: season 1's order is only 12 episodes. Don't know if they'll get a back nine for the rest of the season or if 12 episodes is it. Also, don't know how many episodes next season will be. If it's just 12, they're following the cable formula now for dramatic series.
I don't want it to be the girl because she's sick. She could die off and solve the ethical problem. That's too easy. If Rick discovered it was her and withheld medicine so she died ... well that would turn Carol to the dark side.