Well as we later learn in the comic series.... HUGE SPOILER(S) about the comic but it answers just that.
Click here to see the hidden message (It might contain spoilers)
We learn at the end of one of the trades that EVERYONE becomes a Zombie when they die. In the comic Shane gets shot through the throat on a hunting trip when he tries to shoot Rick in the back. Carl shoots him. When they later get to the prison. IIRC Hershel or someone else had twin girls and a prisoner cuts off their heads and the heads come back to life- Rick immediately leaves and goes back to where the buried Shane and digs up the body. And sure enough. He's come back too. When Risk gets back he's almost a completely broken man and gives the speech that stuck with me throughout the comic series. WE ARE THE WALKING DEAD!!!!