How did Shane and Randall turn so fast?

I think your hypothesis is correct.

Going by what was said on the after-show, The Talking Dead, and the general consensus on the official site from AMC, we are indeed being presented with a plot device that sees everyone being infected with the 'zombie' virus, which is then activated upon death.

Yes, you can be turned from a bite or bad scratch (or at least the turn is accelerated), but now that just dying has been thrown into the mix, it appears that head shots are the way to go for everyone :)

I feel that I have just confirmed what you already thought, but I hope this is of help.

Yes, they turned because they are already infected. Everyone is infected, regardless of being bitten or having an open wound. The belief is that the virus went airborne, meaning it's transmitted via respiratory infection vectors. They breathed in the virus through lung tissue.

Why they turned so quick seems to be due to individual biology, maybe immune system responses. In the CDC, the scoobies see a MRI of a person, alive, but dying due to meningitis like infection of the brain by the Zombie virus (this contradicts the whole zombie virus doesn't kill you thing, but meh). The patient dies. Then the Zombie virus kickstarts the brainstem. The corpse is shot through the brain.

Notice the time code! It goes from living, to dead, to reanimated, to bullet in the brain in MINUTES. 4 minutes .

Other walkers are seen to turn just as quick. Shane reanimates in what seems like a minute. That's more Plot convenience than a realistic attempt at science, but given 4 minutes is shown to be a canon, scientifically proven timing, its believable as a standard deviation.

Before Dr. Jenner commuted suicide at the CDC, he told Rick that everyone is infected. I think that sometime during the apocolypse, the virus became airborne, maybe due to the growth of dead walkers piling up, or something else. The virus became part of the air, and as the survivors inhaled, the virus, along with oxygen and carbon dioxide, traveled through their bloodstream, spreading throughout their bodies. But, in order for the virus to take over the bodies of the survivors, they have to either die naturally, die in the hands of someone else, or get bitten or scratched by a walker.

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.

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 stabbed by Rick, he rises as a zombie because no trauma was dealt to his 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.

No one realised, about all the mix of zombie blood in the parking fight? Shane and Rick got cover their wounds with zombie Blood, this imo count as infection factor Using the Knife, to lure the zombies at the gate also counts.

Old thread, but my thought is that yes, everyone is infected and will turn when they die (unless they have significant head trauma).

The character's fear of a zombie bite is probably down to the stress of the pain and trauma involved, just like avoiding an attack by an animal.

The zombie bite is no doubt dangerous as it's like any other wound, possibly worse as it contains bacteria and infection that will accelerate death, which in turn leads to zombification, but no more than being shot or stabbed (below the neck).

TLDR; they are already infected, but being bitten by a zombie will kill you quickly because they are dirty.