Palavra de Deus:
Robert Kirkman - o criador do Walking Dead franquia, escritor de histórias em quadrinhos e romances e produtor executivo de ambos os programas de televisão - nos deu tantas explicações quanto provavelmente obteremos:
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
E:
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
E:
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
A mecânica de morrer de uma mordida:
No final da primeira temporada, TS-19Jenner, do CDC, explica como e por que uma pessoa mordida morre:
Jenner: It invades the brain like meningitis. The adrenal glands hemorrhage2, the brain goes into shutdown, then the major organs. Then death.
Juntando as peças:
Combinando as explicações de Kirkman com as falas de Jenner da série, eis o que temos:
Todo mundo já está infectado com o que transforma cadáveres em zumbis, mas isso não parece afetá-los.
Quando um zumbi o morde, ele o infecta com patógenos mundanos (isto é, não-zumbificadores). A menos que você seja mordido e amputado rapidamente, você morrerá por perda de sangue (como Amy), infecções mundanas (como Jim) ou ambos (como Tyreese)3, possivelmente).
É difícil reconciliar o ponto #2 com a primeira citação de Kirkman, mas aqui está minha melhor tentativa: quando você estiver quase morto devido à perda de sangue ou infecção mundana, o "vírus" zumbi (ou o que for) que tem colocar movimentos adormecidos em seu cérebro, causa hemorragia nas glândulas supra-renais e o cérebro começa a falhar. Órgãos vitais começam a se desligar e, eventualmente, o cérebro sofre de oxigênio e morre4.
Após a morte, o cérebro fica completamente inativo ou por um tempo. Então o "vírus" zumbi (ou o que quer que seja) reativa o tronco cerebral, e o corpo se levanta e começa a andar por aí, procurando pessoas para morder. O resto do cérebro permanece morto.
Especulações sobre por que as mordidas são sempre fatais (exceto uma rápida amputação da área picada):
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
Embora seja um recurso criado pelo usuário e, portanto, não canônico (e foi por isso que eu o releguei para a seção "Especulação"), o TWD Wiki tem uma explicação muito plausível:
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
A primeira metade da quarta temporada de The Walking Dead dá alguma credibilidade a essa teoria. Esses episódios contam a história de como os sobreviventes na prisão lotada lidam com um surto de algum tipo de doença; um dos porcos de Rick morre com a doença, embora não esteja claro se o porco transmitiu a doença aos sobreviventes ou 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
Nota: Ativado Mortos-falantes, o produtor Greg Nicotero disse especificamente que a doença era não Gripe suína. Isso não é particularmente importante, é claro - tudo o que precisamos saber é que a epidemia nas prisões é uma doença normal que seria fácil de tratar e não especialmente perigosa no mundo pré-apocalíptico.
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
Do roteiro da quarta temporada, episódio 2, Infectado:
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.
Portanto, a doença que matou tantas pessoas na prisão era algo que temos no mundo real, mas era longe mais mortal do que seria para nós. Dos que foram infectados com a "gripe" ou o que quer que seja, sabemos apenas de duas pessoas - Lizzie e Glenn - que se recuperaram; mesmo aqueles dois só se recuperaram após a administração do medicamento antiviral. Pelo menos 28 pessoas6 - e um porco7 - morreu da "gripe" e pelo menos mais duas pessoas infectadas com a "gripe" foram mortas antes que a própria "gripe" pudesse matá-las.
Parte do aumento da mortalidade entre os infectados está relacionada à falta de suprimentos médicos, mas parte disso pode seja o "vírus" zumbi (ou o que quer que seja) faz enfraquecer o sistema imunológico.
notas:
1Omiti isso da citação acima, porque spoilers. O texto que falta é:
...like the Saviors attacked with...
2Aliás, isso pode nos dar algumas dicas sobre o "vírus" zumbi (ou o que quer que seja):
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
3No programa - sua morte nos quadrinhos foi bem diferente.
4Por fim, a maioria das pessoas morre porque o cérebro é privado de oxigênio, e "causas de morte" são basicamente explicações de porque o cérebro de uma pessoa em particular não estava recebendo oxigênio.
5A primeira citação na minha seção "Palavra de Deus".
6Pelo menos pessoas 10 da comunidade prisional e pelo menos três pessoas desconhecidas de algum lugar próximo à prisão. Não sabemos quem eles eram ou de onde vieram, mas esses caras apareceram do lado de fora da cerca da prisão e claramente morreram da mesma "gripe":
Havia também um grande grupo de pessoas que morreram de "gripe" no hospital veterinário de S4E4, Indiferença; essas duas capas de tela são de cenas em que aproximadamente 15 esses "zumbis da gripe" aparecem:
7DESCANSE EM PAZ, Violeta. Você era linda demais para este mundo.