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To me it seems that the mechanism by which zombies infect others is self defeating.

Starting with the classic "Romero" zombies, the zombies want to eat brains. However, in order to kill a zombie, you need to destroy the brain. So, every human that undergoes a "complete" zombie attack will be incapable of being a zombie, because the brain will have been destroyed by being eaten.

In The Walking Dead, the zombies seem to attack with the intention of consuming as much of the victim as possible. Assuming they prefer live flesh to dead, then at first there would be more humans than zombies, meaning attention would constantly turn to the next available living person (though given a human could stay alive during an attack, that could be a while).

However, after a point, the number of victims gets reduced to the point where the zombies would swarm and consume individuals almost entirely (as was done to the horse in the first episode). Lots of dead people, but the number of new zombies would taper off.

Also note that in the second season, when Shane wanted to get away from zombies pursuing him, he shot Otis so that the zombies would turn their focus to the guy who couldn't run, indicating that they will focus on consumption more than pursuit, which reduces the chances of spread even more.

In a scenario of "live" zombies, like 28 Days Later, the infected only seem to attack until the person being attacked shows symptoms. This might be the most plausible form of infection, but unlike undead zombies, it seems to me the people would sustain broken limbs, gouged eyes, and fatal wounds, hindering their ability to really become the threat that they are depicted as being.

It seems that the zombie infection spreads only through incomplete attacks, which doesn't seem likely to spread rapidly. The more a zombie is successful, the less likely they are going to create a new zombie.

Is there some generally accepted rationale that can explain this away? Or am I mistaken in some part of my understanding of how the vectors of a zombie infection would propagate?

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    +1 This is an excellent question. But a really minor nitpick: Romero's "living dead" don't want to eat brains. They seek to consume the flesh of the living, much like in The Walking Dead. The brain thing is an invention of a different series.
    – Andres F.
    Commented Sep 5, 2013 at 15:48
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    Note that this question was closed, yet there is no clear consensus as to whether closing it is appropriate. Please see this meta discussion. Voting to reopen, as closing it was premature without an appropriate meta consensus.
    – Beofett
    Commented Jun 8, 2016 at 13:14
  • Isn't the zombie apocalypse a plague? I think the idea behind plagues is that they end up killing everyone involved.
    – Misha R
    Commented Dec 6, 2016 at 14:23
  • I didn't watch the show, but in the first Walking Dead game a dead body that hadn't been bitten turned into a zombie. Zombie behavior doesn't impact spread
    – aherocalledFrog
    Commented Jul 21, 2022 at 17:44
  • Yes, in twenty-eight days.
    – Mazura
    Commented Sep 17, 2022 at 23:20

7 Answers 7

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You raise a good point. The obvious answer is that partially-successful zombie attacks are in fact the most likely outcome, until large hordes of zombies exist. Killing a single (resisting) human with bare decaying hands is extremely difficult. Injuring or biting one during the fight, however, is very easy.

The prevalence of broken limbs or gouged eyes among zombies is unlikely to be important; the major advantages of the zombies lie in their propensity for surprise attacks and their immunity to pain. Without the effects of shock and blood loss being a factor, the zombies' injuries would not diminish their capabilities as much as you suppose.

However, good science demands a more careful examination.

Medically speaking, a zombie outbreak is similar to any other infectious disease, with an unfortunate extra complication in that deceased victims remain infectious, and mobile. The epidemiology that results has in fact had extensive scientific study (mostly because mathematicians are frequently SF geeks too).

The leading expert in this field is almost certainly Robert Smith? at U Ottawa; I recommend Munz, Hudea, Joe Imad, Smith?: Mathematical modelling of an outbreak of zombie infection as a definitive paper on this subject.

(Those wishing to remain optimistic in the event of zombie outbreak should avoid reading the literature; the conclusions are not encouraging.)

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    @DaveMG: I was being facetious there; if I really thought any answer was obvious I wouldn't bother to answer it. "Least complex" answer is probably more accurate.
    – Tynam
    Commented Dec 26, 2011 at 14:43
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    @DaveMG: I agree that it's not a 'goal' of zombies as such to infect, but it's a likely outcome whether it's the zombie's aim or not. ("Do zombies have goals?" suddenly seems like another question...)
    – Tynam
    Commented Dec 29, 2011 at 23:48
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    @DaveMG: Ah, I understand now. I'm not arguing that a zombie will give up before total victim death; it won't. My argument is that it's far more likely for the human to be injured but escape successfully than to be killed. It's very hard to incapacitate a human quickly with your bare hands (unless zombies are capable of martial arts). If the human is surrounded or restrained, or their first injury prevents escape (say, a broken leg), the human will die. Otherwise the human is likely to escape (with scratches or bites, hence infection).
    – Tynam
    Commented Dec 30, 2011 at 11:18
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    @DaveMG: Thank you. I agree that there comes a 'tipping point'; a human has an excellent chance to escape a single zombie, but crowds are another matter, especially in urban areas where eventually there will be nowhere to run.
    – Tynam
    Commented Dec 31, 2011 at 0:35
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    The PDF has an unfortunate flaw: it does now allow any way to kill the zombies. Any "killed" zombies are put in the "Removed" pile, and new zombies are created from that same pile. Since the PDF claims you can't eliminate zombies, it logically follows that the humans are utterly doomed if there is even the slightest chance of a zombie killing or infecting a human. A more complete analysis would allow some percentage of human-zombie encounters to eliminate the zombie entirely.
    – PotatoEngineer
    Commented Aug 30, 2013 at 19:06
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To be a bit graphic, the human stomach only holds so much meat. A single zombie, or even a small group, is simply limited to consuming an amount of flesh that will result in a fairly complete zombie.

Most (non-Romero style) zombies eat skin or the lower digestive tract, simply because they are relatively accessible (plus a good gut-pulling special effect is always fun to watch!). Zombies do not need a full digestive system to function, and more vital organs (brain, heart, etc.) are protected by bone. This doesn't guarantee that those organs will be unmolested, but it does make it more likely that the attacking zombie will become satiated on less vital portions.

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    Anyone who's seen a child learn to walk can tell you that without your abdominal muscles, walking doesn't happen.
    – Jeff
    Commented Dec 26, 2011 at 14:26
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    And anyone who's seen a single zombie movie can tell you that a crawling zombie can infect a person just as much as a walking zombie.
    – Beofett
    Commented Dec 26, 2011 at 17:37
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    True. That said, I've never known a zombie to become satiated - they ALWAYS eat, even if they physically can't consume more without bursting.
    – Jeff
    Commented Dec 26, 2011 at 17:58
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    Usually the scene shifts after only a minute or two. I can't recall ever seeing a zombie burst, or even vomit, from over eating. Got any examples?
    – Beofett
    Commented Dec 26, 2011 at 19:14
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    Almost all scenes cut away from an attack, which avoids the whole question of when they stop attacking and why, which is the crux of the issue here (as per comments on the other answer). However, while trying not to be too gross... assuming a zombie isn't actually digesting the people it's eating (since zombies break most energy/physics laws anyway), we could assume that anything consumed "passes" right through the zombie, meaning they are never satisfied. If this is the case, though, we're back to not having a good reason why a zombie would stop before completely devouring a victim.
    – Questioner
    Commented Dec 30, 2011 at 5:43
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I think most zombie outbreaks are indeed self-defeating. Even if the zombies don't try to completely consume the bodies of their victims, simply running out of non-zombified humans would be enough. And they seem to go through their supply of humans pretty fast!

The worst case scenario for outbreak propagation seems to be 28 Days Later, where even though the infected don't consume their victims (as you noticed), they seem to simply run out of energy and die off after a while. This is because they are living, not undead, and therefore not "true" zombies. In a way, they are simply hostile human patients who resist medical treatment.

Note however that you are mistaken about how the infection spreads in both Romero's Living Dead series, and in The Walking Dead:

In both cases, anyone who dies becomes a zombie. So bite or no bite, any person dying of whatever cause, whether zombie-related or not, becomes a zombie. And in the case of Romero-style undead, we never know what causes them, whether a virus or a supernatural cause.

So there is no traditional "propagation" or "vector" in these cases.

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There is a good enter comic on this from SMBC about the ineffectiveness of zombies looking like horrible monsters and devouring people. I would embed it, but it's a rather long one!

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    But you could paraphrase it.
    – Angelo Fuchs
    Commented Sep 25, 2015 at 10:30
  • I can't paraphrase it (what the opposite of paraphrase?) but's that comic is basically paraphrasing Richard Matheson's I am Legend, 60 or so years later :)
    – Binary Worrier
    Commented Jun 7, 2016 at 13:26
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Well, it's not just a bite that infects someone. In a book I have, part of a series called ZOMBIE APOCALYPSE! - Horror Hospital, the infection can be spread through a bite, scratch, ingestion of bodily fluid - whether that be getting zombie blood in your mouth when fighting them off or kissing a newly infected before they start to show symptoms - or, and this is even worse than the last one, by getting bitten or swallowing one (or more) of the red flees the undead spew before dying for good.

In this series, the zombies have one primary instinct; spread the disease. Only once a lot of zombies were ambling around did they start attacking with an aim to feed. Also, they can become more intelligent through eating the brains of humans, and eventually a human/zombie war breaks out with the end goal being to wipe out humans. As regards to the zombies sustaining wounds, they deliberately infect doctors who discover that zombies can be pieced together from bits of the truly dead.

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Zombies lack coordinated movement so a lot of attacks may fail leading to an infection with zombieness or zombiety.

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Not really because the Romero zombies don't eat brains at all (probably because the brain is protected by the skull which is hard to get through) They simply attack and kill the victim which will later become another zombie

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