In the TV show The Talking Dead, they explain it as the zombie virus (also known as the walker virus) being airborne. This simply means that if a person ever gets an open wound such as a cut on the face or hand or anywhere else on the body, you already have the zombie virus in you.
So when Carl was shot, that means that zombie virus got in him through the gunshot wound but he wasn't turned over to a walker because he never died from the wound. On the other hand, both Rick and Shane had wounds on their hands from the previous episode when they cut open their hands to attract the zombie to the gate and to the opening in that bus door which means that the walker virus was put into both of them. Shane was changed over because if you noticed he had no wounds directly to the brain, only the heart which would allow him to "rise up" as a zombie.
In the episode where the group that is lead by Rick goes to CDC, in the video it shows that the zombie virus only effects the brain stem making it reanimate leaving the rest of the body cold and dead. So if the virus only effects the brain then that means the only way you can actually be killed is if you are shot, stabbed, slashed, or have anything slam through the skull puncturing the brain. If you are shot through the head, bit in the neck, get your neck broke or anything within the category of injuries that would kill a normal non-changed human being you would still be able to "rise up" from the dead as a zombie/walker/crawler.
So the only way you can actually be killed and remain as a dead corpse is if you are shot through the head or if you have any sort trauma to the brain that causes it to be punctured.