I'm probably wrong but i had the thought that earlier in the season they pulled a walker out of a well maybe he was infected this way..
Good point! I thought when Shane shaved his head after he killed Otis that night that maybe he could become a Walker. I saw (I thought) a scratch on his head. And then he did look and start acting kind of weird/angry/violent at that point. AND to top it off, we know he was probably disappointed (to a degree) that Rick was alive and destroyed his little set-up with Laurie.
Anyway, can't wait to see more.
TIS