Walking Dead pathogen transmission method

Question

In The Walking Dead (I am familiar only with the tv show), when one gets bitten by a zombie, that person becomes a zombie. But do you need to be bitten in order to be dangerously exposed? Are there other methods of transmission?

In episode S2E10, it is briefly theorized that two dead walkers became infected not through bites, but through scratches.

A lot of the zombie action is up close and personal. It is not uncommon for the humans to leave the melee with blood all over them, including on their face. Would they be exposed if any zombie's blood got on an open cut or in their mouth or on their eyes?

Answer 1

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.

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You may be interested in these questions:

• Why are Walker bites in the Walking Dead fatal?
• Do the survivors in The Walking Dead have immunity to the zombie plague or are they just lucky?

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?

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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.

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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 "virus". Source

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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

Answer 2

In the comic book, how people become zombies is well established:

The zombies are the classic slow walking Romero style zombies, which originated in Night of the Living Dead. Following classic Romero-rules, everyone who dies, for any reason, will re-animate as a zombie. Even someone killed by a gunshot wound with no zombies involved at all will re-animate as a zombie. Zombie bites are not how the "infection" is spread. Instead, zombie bites and saliva cause infections that are 100% lethal. Characters have speculated, but cannot confirm, that whatever causes zombies to re-animate is some sort of virus that everyone in the world is technically infected with but is only triggered by death. It is known, however, that upon death, it can take anytime between three minutes and eight hours for the corpse to reanimate.

Source: Zombies - Walking Dead Wiki

Robert Kirkman wrote in Issue 41, page 29, "Letter Hacks":

"...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 "virus.""

At the start of season 2, there was some evidence that the TV show may not be following this rule from the comic:

Interestingly, during the second season of the TV series, as the group is stopped on the highway there are multiple dead and decayed bodies that have not reanimated. The reason for this is unknown.

Source: Zombies - Walking Dead Wiki

The question brings up "18 Miles Out" where

Rick and Shane discover two zombie policemen who have no evidence of of bitemarks. Rick and Shane are both under the assumption that bites turn you into a zombie, so Rick guesses that perhaps scratches caused the transformation. This is not proof of scratches causing anything. This is just Rick guessing based on not much evidence.

However, in the episode "Better Angels"

both Randall and Shane die without being bitten or having any contact with walkers, yet both come back as zombies.

Given this development and its confirmation at the end of the season two finale, it can be safely assumed that the TV show is following the same rules as the comic.

Answer 3

Jenner whispered something to Rick, and it is later revealed that Rick was told that everyne was infected. The context of what Jenner said needs to be considered. He could very well have been talking about Ricks group specifically, not the whole of humanity. He did perform blood tests on everyone when he allowed them into the CDC complex. Considering the fact that the group has been in direct contact with walkers on very frequent occasions, it is perfectly reasonable that every one of them could have become infected by conventional means. The opportunities have certainly been there.