[GothingNC]

The part about Rick not hearing Shane raising back up bugged me too.
Rick may have been in shock after killing his friend and then seeing his son pointing a gun at him.

I am thinking everyone is now a carrier of the virus after being exposed from the splattered zombie blood and the ash from the burning bodies contaminated the soil/air.

Shane may have been going though symptoms of the virus toward the end after being exposed to the walkers' blood on his sliced hand while trapped in the school bus.