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In this answer it is pointed out that everyone appears to be infected with the virus. As we saw in the CDC center, death triggers the virus to reactivate portions of the brain.

Yet even a single bite from a Walker results in fever, eventual death (within days), and then reanimation.

If the Walkers are infected with the same virus that is already latent in the survivors, why do bites seems 100% fatal?

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Additionally, why wouldn't being drenched in and possibly ingesting/inhaling a walker's blood or getting in your eyes result in the same effect? – Force Flow Oct 19 '12 at 20:58
    
A fever starts when the body is trying to heal itself/ fight an infection or unknown bacteria/virus of some sort.. When the body is focusing on getting rid of this, it goes into a hard working process. After a while you'll get a fever - as a symptom that the body is battling against the unwanted "guest". – user10548 Oct 30 '12 at 7:40
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Don't feel like writing a full answer, but I believe you also have to remember that they already have a weakened immune system so when they get bit it will introduce new bacteria and will cause an infection. So I believe they die from an infection and as a result, turn into a walker. – EricSSH Aug 3 '14 at 0:07
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Placebo effect. If you think being bitten by a zombie turns you into a zombie, it will. ;) – Theik Apr 7 '15 at 9:56

10 Answers 10

up vote 3 down vote accepted

Word of God:

Robert Kirkman - the creator of the Walking Dead franchise, writer of the comics and novels, and Executive Producer on both television shows - has given us as much of an explanation as we're likely to get:

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."
- Robert Kirkman, The Walking Dead, Issue #41, Letter Hacks

And:

Just to get this on record once and for all - and it is complicated, I know - here's how zombification works. Whatever makes people come back as zombies after they die - it's inside them. It's inside everyone. No matter how anybody dies, as long as the brain is intact, they turn into a zombie.

So what the fuck does a bite do?

Well, bites, and direct to blood contact with zombie gunk, [...1] causes death. It's a strong infection that leads to fever that kills someone. Then the "virus" or whatever is already in them, turns them into a zombie.
- Robert Kirkman, The Walking Dead, Issue #146, Letter Hacks

And:

Q: This is a real Comic Book Guy question – but do I gather that if someone swallows a tiny bit of zombie blood they won’t turn into one of the undead? There was a lot of it being sprayed around this episode [Season Three, Episode 6, Hounded].

A: Yeah, people to a certain extent think of zombie blood as being like the blood from Alien. You know, in the Alien movies it’s like, “Oh god, if it touches you, you explode!” or whatever. Whatever it is that turns these people into zombies is in them already. So the idea of getting zombie blood on your face, which happens all the time, and it turning you into a zombie is something that’s just not the case.

Now, that doesn’t make the zombie bite any less lethal. You know, breaking the skin, having that kind of contact with the toxicity that zombie mouths would have, would be something that causes an infection that definitely would lead to your death and then the thing that’s already in you would turn you into a zombie. So there is a science to this, to a certain extent.

Q: Although, to be clear, you are not technically a scientist.

A: No. No, no, no. But I know about everything that scientists know, I’m pretty certain! But anyway, zombie blood is not quite as deadly as a lot of people think. I wouldn’t drink it in high volumes, though.
- Robert Kirkman, interview with Entertainment Weekly


The mechanics of dying from a bite:

In the Season One finale, TS-19, Dr. Jenner at the CDC explains how and why a bitten person dies:

Jenner: It invades the brain like meningitis. The adrenal glands hemorrhage2, the brain goes into shutdown, then the major organs. Then death.


Putting the pieces together:

Combining Kirkman's explanations with Jenner's lines from the show, here's what we get:

  1. Everyone is already infected with whatever turns corpses into zombies, but it doesn't seem to affect them.

  2. When a zombie bites you, it infects you with mundane (i.e., non-zombifying) pathogens. Unless you're bitten on a limb and quickly amputate that limb, you will die from blood loss (like Amy), the mundane infections (like Jim), or both (like Tyreese3, perhaps).

  3. It is hard to reconcile point #2 with the first Kirkman quote, but here's my best attempt: Once you're weakened to near-death from the blood loss or mundane infection, the zombie "virus" (or whatever it is) which has been laying dormant moves into your brain, causes the adrenal glands to hemorrhage, and the brain begins to fail. Vital organs start shutting down, and eventually, the brain is starved of oxygen and dies4.

  4. After death, the brain is completely inactive or a while. Then the zombie "virus" (or whatever it is) reactivates the brain stem, and the dead body gets up and starts walking around, looking for people to bite. The rest of the brain remains dead.


Speculation on why bites are always fatal (barring a quick amputation of the bitten area):

At the end of season one of TWD, protagonist Rick Grimes discovers that everyone carries a virus which “reboots the brain” upon death, resulting in a ghastly transformation. But if it is merely death that turns you, why do all of the characters regard a zombie bite as a death sentence? Why is it one of the first questions they ask strangers, and why did Herschel need to lose a leg?

There is a way to square this with Kirkman’s quote5, and more interestingly, with science. Dead bodies can be dangerous in and of themselves, so humans have always taken precaution with them. In fact, in a case where we have to handle corpses, such as after a natural disaster, there are a number of steps responders have to take to avoid infection and disease. For example, a rotting body can still transfer gastrointestinal pathogens, tuberculosis, and hepatitis to the living. So, one can imagine that a biting mouth of a rotting corpse, continuously chomping down on humans, isn’t the most hygienic place.
- Scientific American: The Komodo Dead

Although it is a user-created resource, and therefore not canonical (which is why I relegated it to the "Speculation" section) the TWD Wiki has a very plausible explanation:

Everyone in The Walking Dead universe somehow contracted the pathogen that, for reasons and through means unknown, brings the dead back to "life." It is unknown how the disease is spread, though its apparently total infection rate worldwide suggests it is either water-borne, air-borne, or both. The exact taxonomy of the pathogen is unknown. The pathogen itself does not kill its hosts, but it seems to weaken their immune systems considerably, to the point where even minor illnesses are far more likely to be fatal than normal to humans.

As previously stated, the zombie pathogen itself is not lethal, and the zombie "infection" occurs due to pathogen weakening the host immune system. This makes bacteria found in zombies, especially in their mouths, that much more lethal than they normally would. Nevertheless, the pathogen has two separate but parallel modes of infection: latent and fluid contact/bites/scratches.

Zombie bites are not necessarily fatal because of the zombie pathogen. One possible explanation is that, through bites, the pathogen induces a rapid immune system response that accelerates the onset and severity of symptoms caused by bacteria in the zombie's mouth.
- TWD Wiki

The first half of the fourth season of The Walking Dead lends some credibility to this theory. These episodes tell the story of how the survivors in the crowded prison cope with an outbreak of some sort of disease; one of Rick's pigs dies from the illness, although it isn't clear whether the pig transmitted the disease to the survivors or vice versa.

Q: I’m not a doctor – I just play one on TV – but I’m assuming there’s some sort of nasty, swine flu-type virus abroad in the prison.

A: Well, it’s definitely some kind of nasty illness. Something that can make a guy go from "cough-cough-I’m-not-feeling-well-can-I-go-lay-down?" to essentially bleeding out in the shower after a few hours is something on the swine flu level – possibly a little bit more deadly.
- Robert Kirkman, Entertainment Weekly interview

Note: On Talking Dead, producer Greg Nicotero specifically said the disease was not Swine Flu. That isn't particularly important, of course - all we need to know is that the prison epidemic is a normal disease that would be easy to treat and not especially dangerous in the pre-apocalyptic world.

It’s important to know that this is really just some form of the Spanish Flu. It’s something that exists in our everyday world now.
- Robert Kirkman, Vulture interview

From the script of Season Four, Episode 2, Infected:

Rick: No bites. No wounds. I think he just died.

Dr. Subramanian: Horribly, too. Pleurisy aspiration.

Herschel: Choked to death on his own blood. Caused those trails down his face.

Rick: I've seen them before on a walker outside the fences. I saw them on Patrick, too.

Dr. Subramanian: They're from the internal lung pressure building up - like if you shake a soda can and pop the top. Only imagine your eyes, ears, nose, and throat are the top.

Rick: It's a sickness from the walkers?

Dr. Subramanian: No, these things happened before they were around. Could be pneumococcal. Most likely an aggressive flu strain.

Daryl: How could somebody die in a day just from a cold?

Rick: I had a sick pig, it died quick. Saw a sick boar in the woods.

Hershel: Pigs and birds. That's how these things spread in the past.

So the disease that killed so many people in the prison was something that we have in the real world, but it was far more deadly than it would be to us. Of those who were infected with the "flu" or whatever, we only know of two people - Lizzie and Glenn - who recovered; even those two only recovered after antiviral medication was delivered. At least 28 people6 - and one pig7 - died of the "flu", and at least two more people infected with the "flu" were killed before the "flu" itself could kill them.

Part of the increased mortality among the infected is related to the lack of medical supplies, but part of it may be that the zombie "virus" (or whatever it is) does weaken the immune system.



Notes:

1I omitted this from the quote above because spoilers. The missing text is:

...like the Saviors attacked with...

2Incidentally, this might give us some insight into the zombie "virus" (or whatever it is):

In at least 50% of cases, bilateral adrenal hemorrhage is associated with an acute, stressful illness (eg, infection...) or event (eg, surgery or invasive procedure).

Infections associated with extensive, bilateral adrenal hemorrhage are diverse; they include sepsis, wound infections, pneumonia, pseudomembranous colitis, influenza, varicella, and malaria.

Waterhouse-Friderichsen syndrome (purpura fulminans) represents hemorrhagic necrosis of several organs, including adrenal hemorrhage, in the setting of overwhelming sepsis. The syndrome frequently is characterized by a distinctly hemorrhagic skin rash. Although Waterhouse-Friderichsen syndrome originally was recognized in association with meningococcal disease, which still accounts for 80% of cases, the syndrome also has been associated with other bacterial pathogens, including Streptococcus pneumoniae, group A beta-hemolytic streptococci, Neisseria gonorrhoeae, Escherichia coli, Klebsiella pneumoniae, Haemophilus influenzae (group B), Salmonella choleraesuis, Pasteurella multocida, Acinetobacter calcoaceticus, and Plesiomonas shigelloides.
- E-Medicine

3On the show - his death in the comic books was quite different.

4Ultimately, most people die because the brain is deprived of oxygen, and "causes of death" are basically explanations of why a particular person's brain wasn't getting any oxygen.

5The first quote in my "Word of God" section.

6At least 10 people from the prison community, and at least three unknown people from somewhere close to the prison. We don't know who they were or where they came from, but these guys showed up outside the prison fence, and clearly died of the same "flu":

enter image description here

There was also a large group of people who had died of the "flu" at the veterinary hospital in S4E4, Indifference; these two screen caps are from scenes in which roughly 15 such "flu-zombies" appear:

enter image description here

7R.I.P., Violet. You were just too beautiful for this world.

enter image description here

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There's several possible reasons.

Firstly, human mouths are dirty. A bite from a normal human is likely to become infected in many cases even today, when we have over-the-counter disinfectants, bandages, etc on hand in almost all locations. Left untreated, this alone could be fatal. This is unlikely today, in modern society, but not nearly as unlikely if society has broken down and medical treatment is unavailable. This is discussed to some degree in The Zombie Survival Guide by Max Brooks, which is based (as is TWD) on Romero-style zombies.

Next there's the reanimation. The virus is dormant, as you say, until death. If the zombie has any form of saliva (which they seem to) or moisture in their mouth (without which verbalizing would be nearly impossible) their bites will almost certainly introduce the live, active virus into the bitten person's bloodstream.

There's a HUGE difference between a dormant virus and an active one.

The active virus seems to cause fever, death, and reanimation. The dormant virus activates upon death, causing reanimation. It's also possible that the dormant virus activates upon contact with an active sample of the virus, which would help account for the rapid onset of symptoms.

In either case, amputation could potentially save the life of someone bitten upon a limb - if the active virus doesn't have time to make it to the heart, it can't propagate (either normally or by activating the dormant virus) throughout the rest of the body.

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+1 to this very well-thought out answer. I've popped positive as a PPD converter, which means that I carry a dormant variety of the bronchitis virus. Which is obviously not the same as the active variety, lest I be quite dead. – Terrance Shaw Oct 19 '12 at 18:42
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@Jeff: Your theory about amputation was proven right when they amputated Hershel's leg in the show. Nice answer! – JNat Nov 16 '12 at 13:04
    
@JNat: I'm glad the show did that. That said, I cheated: I've been reading the comments for years, and it works in there. Couldn't be 100% sure the show would follow that, though. – Jeff Nov 17 '12 at 14:44
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I'm pretty sure it's not the saliva. Otherwise very rotten corpses, with no working saliva glands, would be harmless. I'm thinking it cannot be blood either, because at one point Michonne is soaked in walker blood, but also has a badly patched stomach/side wound of her own, and she isn't worried about getting infected (and she doesn't). – Andres F. Feb 25 '14 at 17:18
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@AndresF. They may not have saliva, but without some form of moisture in their mouth and throat, the zombies couldn't groan or moan. It's possible that heavily decomposed walkers will have the liquified residue of decomposition within their mouths. – Jeff Feb 25 '14 at 18:53

I am afraid that attempting to explain this using real world science will always fail. The idea that zombies can be caused by viruses is completely absurd despite its popularity. A virus needs living cells in order to be able to reproduce. Therefore, if the host cells have stopped reproducing (as is the case upon death), the virus will be completely incapable of doing anything at all. I might be willing to suspend my disbelief to accept, for example, bacteria or any other pathogen doing this but a virus is just silly. Pop-science just happens to find viruses cooler is all.

A dormant virus is simply one that is not currently hijacking the host's cellular machinery to make more copies of itself. OK, that explains the survivors. However, having the virus activated by death is, while theoretically possible (see, I can too suspend my disbelief!), very futile. The virus will wake up, try to replicate itself and fail because the cells are dead.

Since we know that zombies do not heal, we also know that their cells are dead despite their tissue being animate which means that a virus would simply not function. Therefore, all bets are off when it comes to explaining this using actual science. We have to posit that viruses are a completely different thing in-universe to what they are outside it and all "scientific" discussion is moot.


Full disclosure: I'm a biologist so I tend to take umbrage at such blatant disregard for fundamental scientific facts :)

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An alternative explanation to viruses being seen as cooler is that they are currently seen as more difficult to understand and cure by today's medicine than bacteria. A virus based zombie infection then seems more deadly and terrifying. – JBentley Jan 14 at 12:48
    
As for realism, cells do not die immediately when the body dies. There is some time lag. The (now activated) virus could use that time to hijack any cells which are still alive, then keep just the right subset of them functioning to provide the abilities of a zombie but not that of a normal person. Including repairing parts of the body that did die. We could imagine it as being the result of some failed experiment to create a virus that defeats death. – JBentley Jan 14 at 12:56
    
@JBentley actually, viruses are far, far better understood than bacteria. They are the simplest "life form" we know of, after all. What makes them hard to kill is precisely that simplicity. They have no essential functions to disrupt since they hijack our own cells. The hard bit is to kill the virus without killing us. – terdon Jan 14 at 13:22
    
@JBentley nice idea about hijacking the living cells but I very much doubt that would be possible. Yes, some cells might remain alive for a short time until they run out of resources. However, reproducing takes a hell of a lot of resources and a virus needs a cell that can reproduce in order to make more copies of itself and infect the host. So, these cells wouldn't be able to reproduce (or, at least, certainly not enough to make the necessary number of viruses to infect other cells) despite still being alive. Not without access to the body's supply lines. – terdon Jan 14 at 13:25

I don't agree with Jeff. There is a difference between a virus in a latent cycle and an active one, but not to the extent or manner Jeff describes; at least in not in regard to immunity.

I'm of the opinion that the Survivors aren't immune to the virus, meaning they are not immune in the sense that the virus still replicates inside them to some extent.

From the show we know that virus causes a deadly fever following which the corpse is re-animated. Death of the host seems to be necessary for the virus to create a 'walker'. It seems more like the survivors are "immune tolerant" of the infection. Meaning, they are infected and the virus replicates inside them at some level, however their immune systems tolerate this infection as opposed to inducing the deadly fever in an attempt to squash replication.

In this way, they could seem immune when really they are just not suffering from the deadly fever which is the necessary precursor to becoming a walker.

It is fairly common for the viral infection itself to not be the cause of disease. In the real world, Ebola, Dengue, and West Nile all cause disease indirectly by stimulating an over-reaction of the immune system. Indeed, many potential treatments for these diseases include immune suppressants to lessen the severity of the patients reaction in addition to direct antivirals.

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Very interesting. However, how does a bite trigger the fever and subsequent death in those who are immune tolerant? – Beofett Oct 19 '12 at 16:24
    
"not be the cause of disease" Do you mean "death" not "disease"? – Patrick M Nov 19 '14 at 10:32

A reason could be like a Komodo dragon, which has bacteria in its teeth. If someone bit you with rotting flesh in there teeth you would probably get extremely sick and you might die as well.

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Has anyone considered:

It's two viruses in a symbiotic relationship?

Virus A: Something like an anaerobic rabies that "activates" on death as the oxygen content of the body lowers due to decomposition.

Virus/Bacteria B: And then another flu-like virus/bacteria that can be passed on by saliva to blood (IE you can drink your own saliva and be fine because the virus is too large to be absorbed into your blood stream via your intestines but not too large to be "injected" when you break someone's kin with a bite).

This one kills with a bite, and then the rabies takes over upon death causing you to become someone who bites a lot. Perhaps this is a decomposer bacteria rather than a virus and causes your body to become an anaerobic environment more quickly, further assisting the rabies.

This would also make a hell of a lot of sense from the "genetically engineered virus" point of view that's usually the cause of zombie outbreaks. It might be that only one of the two viruses was intentionally created without people knowing the other existed and thus not knowing they would work together so well, which explains how they could be so stupid as to let this out into the world.

IE: Everyone has Virus A Somehow Bacteria B gets let out into the world (or the natural bacteria of human mouths mutates).

A few people get bitten, they die of Bacteria B, which decomposes their corpse more aggressively,m making their internal systems an anaerobic environment, activating Virus A, they become the undead and bite others.

Or maybe Bacteria B has been knocking about fairly harmlessly somewhere in Europe and all of America is infected with Virus A. The moment someone from Europe visits the USA and bites someone, they become infected with Bacteria B, they die, they decompose internally a bit more quickly than expected, anaerobic environment is established, rabies activates and they are now a zombie that carries both diseases, few people get bitten by "mad man who was declared dead". some of them travel around etc and it goes world wide.

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It's been answered in Letter Hacks (Issue #41)

"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."" —Robert Kirkman

Whatever the science of the virus triggering on death, death by zombie bite is from infection (think of all the bactiria on a corpse and in its mouth; even the healthy human mouth is teeming with it; add the zombie's non-fussy diet and it's bad news). Getting their blood in your mouth or eyes could certainly be hazardous but it depends on a lot of factors; how much, if you have gingivitis or open sores in the mouth or nose, and then what state the blood was in, and your own imune system.

The fever preceding death may be from the virus already in their system gearing up, then simulating death (highly probable; 'hitting restart' to effect the change), or from the infection their imune system trying to fight (seems less likely to me); no one knows, but the bite is not passing on the 'zombie virus', as they already have it.

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I've wondered this myself many times and I've only come up with one viable solution. My theory is that the current survivors when exposed to the initial wave of infection were able to create antibodies quick enough to prevent turning into a walker. The fact that everyone alive today remains infected demonstrates that even those whose immune systems were able to handle the disease/virus/parasite (I don't believe we know for certain yet what this is) are only able to keep the infection dormant, not drive it out completely.

Without any resistance from the bodies natural defenses, I.E. death, the infection immediately begins to spread virulently, multiplying exponentially throughout all the bodies core systems. This means a direct bite from a dead or non-immune person would be extremely concentrated, overwhelming the bodies defenses. The lymphs would swell to enormous size, a very high fever would develop, and the victims blood which quickly become toxic, and the organs would begin to shut down. A painful death for certain.

As for accidental ingestion through the mouth - Presumably despite not being shown on the series or in the comics, most people are maintaining basic dental hygiene. (Much of the day to day detail is not shown and is only implied, and in a world without doctors and dentists, health maintenance is EXTREMELY IMPORTANT) If that is the case, and you don't have dirty gums or lesions in your mouth, its actually pretty safe. It is quite likely the mystery pathogen would be destroyed through the natural initial processes of digestion (naturally occurring disinfectant properties of saliva and the much stronger acidity of the stomach) This might also hold true of the nose and ears. (Your body does a pretty good job of filtering out particulate matter from the air and shunting it down the throat.

This pretty much just leaves the eyes and any open wounds. Keep in mind, that since by and large zombies are EXTREMELY dehydrated, their blood will be very thick and congealed, almost like a thick tomato paste. With a blunt force impact, any blood spray will be directed away from the attacker. Same with a slicing type attack like a sword or machete. The only real danger would come from drawing back a puncture weapon, think knife in the eye, and even then, it'd just be a little short range squirt, and I think people would instinctively shut their eyes while they did it. As for an open wound... well if you get contact with zombie blood on your open wound, I think you're pretty screwed. At that point I think you're praying that your bandage has held a seal...

Sorry for the looong post, but thats the only theory I have that I haven't been able to poke holes in.

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I like how you think, but at this stage we've seen characters almost bathed in walker blood who don't turn. Michonne at one point has a badly patched stomach wound while being covered in walker blood, and she doesn't turn. Compare this with real-life HIV transmission, with health care workers warned to use gloves and not to let skin scratches get in contact with infected blood. – Andres F. Apr 3 '14 at 23:52
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True. Another option could be that the unique body chemistry of the survivors causes the zombie infection to act very similarly to HIV, dramatically lowering the immune system but stopping short of killing them directly, making a zombie bite just an incidental exposure to whatever terrible diseases are thriving in that filthy, moist environment. That could also explain the mortality rate of that plague that spread so quickly among everyone at the prison. Maybe it wasn't a crazy super flu, maybe it was just a cold... – SA295 Apr 4 '14 at 21:03

Here is the virus' life-cycle, as I see it:

Day zero hits, which is the day when some strain of virus mutates into Virus Z. So at this point, our victim, John, doesn't have the virus. Some time after day zero, John catches what he thinks is a common bug. The symptoms John feels are his immune system responding to the infection of Z. Since Z is incapable of compromising a healthy immune system, Z goes into dormancy as a defense mechanism. While dormant, Z does not replicate, and hides within the host's own cells until reactivated by some specific stimuli (this is known as viral latency.)

So John, who is still healthy, now has a number of dormant Z viruses floating through his bloodstream. One way to reactivate Z is by way of death. Another way to reactivate the virus is via a large immune response to the presence of a strong new infection of active Z. This isn't too much different than how some viruses, like herpes, work; which is thought to reactivate only to very specific immune system responses.

Now, let's say John gets a Walker's blood in his mouth. The number of activate Z viruses in the blood splatter would be relatively small, and aren't nearly enough to compromise John's immune system or create the specific immune response needed to reactivate the dormant Z in his blood. So John is still alive and well.

Finally, poor John gets bit on the arm. He receives a large surge of new Z into his blood stream where the wound occurred. At this point, John has two options:

  1. Accept his fate and allow the flood of active Z to create the exact immune response that the dormant Z has been waiting for to reactivate. Or...
  2. Sever the arm before his immune system can respond to the new, Z infection.

John goes with option #1. We don't know just how much dormant Z John has accumulated since day zero, but we know it must have an incredibly strong presence based on how fast it kills. Ebola can take around 1-2 weeks. Whereas Z can kill in hours. John's immune system is quickly overwhelmed, and he succumbs to the virus.

RIP in peace, John.

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How does this explain Michonne being bathed in zombie blood while also having a badly patched stomach wound? This happens in one of the Prison episodes. This would allow a large amount of "active Z viruses" (to use your teminology) to enter Michonne's bloodstream via her exposed stomach wound. However, Michonne doesn't turn and she obviously doesn't amputate her stomach ;) Therefore, I think she serves as a refutation of your theory. – Andres F. Sep 21 '15 at 16:38

Because the walker had been incontact with the virus and when it bites you the virus will spread from the walker to you. Then the virus will slowly kill you. Eventually turning you in to a walker. Plus there are all infected so even if they don't get bitten. If you die you will still turn.

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