I don't believe what I wrote below is a spoiler but to be safe I will white it out this explains when the living dies they turns into Zombies w/o being bitten....Seems like everybody should know this by now. SPOLIER IN WHITE HIGHLIGHT TO READ in season 1 at the Atlanta CDC building before the building blew up the Dr scientist guy whispered something in Ricks ear ....He told Rick everybody is infected if you die you will become a Zombie the only way to kill the living is a bullet in the head and they wont turn into a Zombie. The Dr. incinerated himself. Season2 Rick was in the bar and shot both of those guys in the head. Rick did not shoot Shane in the head maybe it was the shock of it all and Shane became a Zombie. Rick is afraid to tell the rest of survivors so they dont give up and loose faith.