Everyone already has the virus. If they are bitten by a zombie then it kills them and makes them a zombie. If they die naturally and do not receive a bullet or blow to the head, they will "rise" as a zombie. If bitten, they will turn to a zombie unless the sustain a head blow; then they will become a corpse.
Shane, just like everyone else, has the virus in him. When he gets shot by Rick, he rises as a zombie because he was not shot in the head. Once shot in the head he is a corpse. Randall also had the disease and he was killed by breakage his neck and when was left there "dead" rose as a zombie soon after.
Basically, no one is immune, everyone is sick and there are two ways of being turned; killed without a huge trauma to the head or a bite from a zombie which my theory is increases the disease in the body and accelerates death and immediate zombie transformation. The only way to kill anyone, zombie or "not zombie", is trauma to the head.