From my understanding, it seems that the virus that everyone has is somehow held back while you are alive, probably the body is able to fight it off enough to hold it in remission. Then once you die, either by natural or gun shot or broken neck in randal's case or by bleeding out the body is not able to fight it anymore. However if you get bitten or scratched it directly enters your blood stream and all havoc breaks loose.
Seems to me like when Jenner (sp?) took blood samples he was looking for someone who might possibly have immunity to the virus??
Anyways it also seems that as we progress through the show the "turning time" seems to be shorter. Even though i know jenner said they heard times from 2 minutes to ___ hours it seems that they turn quicker and quick. Amy in the first season took all night and Jim took quite a while, while shane and randal seemed to have turned fairly quickly. It will be interesting to say the least to see what happens to Hershal....
"Speed has never killed anyone, suddenly becoming stationary… That’s what gets you."