In the season 2 finale, Beside the Dying Fire, Rick reveals the following:
Everyone is already infected. Rick admits to the group that Jenner told him this fact but Rick did not believe him 100%. He purposely withheld this information from the group.
You may be interested in these questions:
Since we are only talking about the tv show, we only can say that zombie bites cause a fever, death, and finally reanimation (up to Season 2, Episode 10).
We, as viewers, can only suspect that death through normal means can lead to being reanimated as a Walker. So far we have not seen anything to prove this but the show has left the following hints:
- In the episode, Nebraska, Rick shoots Tony twice in the chest, enough to kill him. However Rick immediately walks up to him and shoots him in the head... WHY?! (link to video)
- In the episode, 18 Miles Out, Rick and Shane come across two slain Walkers that used to be cops or security guards. After Shane examines the bodies, he tells Rick that there were no bite marks present. Rick conjectures that maybe they were scratched and maybe that caused their transformation.
Could the fact that you do not have to be bitten/scratched by a Walker in order to become a Walker be what Jenner whispered to Rick during the last episode of Season 1? Could this be why he tried to convince Shane that maybe the guards were scratched?
There have been a few examples of people getting Walker blood on them, or at least onto a body part where it could have been easily absorbed, but have not caused them to die and reanimate:
- Rick and Glenn cover themselves with walker blood and guts in order to disguise themselves, Guts (Season 1, Ep 2).
- Andrea stabs a Walker and the blood splatters into her eye. Episode What Lies Ahead
- Rick and Shane cut their hands to use their blood as 'bait' in 18 Miles Out. Both have close-encounter fights with zombies but have not yet succumbed to death/reanimation.
This definition was given by one of the creators, Robert Kirkman. Although it probably applies to the comic book definition, some/most/all(?) may apply to the TV series.
...the rule is: WHATEVER it is that causes the zombies, is something
everyone already has. If you stub your toe, get an infection and die
... you turn into a zombie. UNLESS your brain is damaged. If someone
shoots you in the head and you die ...you're dead. A zombie bite kills
you because of infection, or blood loss ... not because of the zombie
What about the bodies we see in cars, how did they die without turning into walkers?
This does raise the question about the dead bodies in the season-opening traffic jam — why hadn’t they all been turned?
I think if you go back and watch that [sequence you'll see] we were very careful to have them be in cars that were in accidents, so the brain would’ve had trauma. Or they had some kind of wounds somewhere on their heads to show that their brains had been killed, like somebody came across and killed them. We knew that we were building to this throughout the entire season.
Source: TVLINE interview with Robert Kirkman