There are many theories. Maybe it was because of the handshake in the beginning of the episode where Darryl licked his fingers before. Or he got sick.I just finished watching it. Didn't really understand what happened to the guy to make him collapse and turn so quickly at the end.
I am mostly critical of this episode. Everyone is falling in love which was a bit unrealistic in my eyes! I also noticed some poor editing near the beginning when they were staking the walkers through the fence - one of the poles went over the zombie's head, but it still died! Hmm....
Anyway, it was a good start but not much happened. Hopefully it will pick up! Season 2 is still my favourite!
In the recap you see Rick saying that if you die you turn, so maybe theres something else to it.. If you get sick maybe you turn.
Anyhow poor kid, he was representing the nerd community of the walking dead :p