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I am wondering what would be the most likely cause of zombies. The movie Night of the Living Dead suggested that radiation may have been the cause for the zombies. But plenty of incidents on Earth have exposed people to varying degrees of radiation, and none of them have turned people into zombies.

Perhaps an infectious agent (such as a bacterium, virus, fungus, prion, or parasite) would be more likely to turn people into zombies. The novel World War Z used a virus as the cause for the zombies, but I'm not aware of any virus on Earth that comes close to turning people into zombies.

Would a prion be the most likely cause of zombies? It seems all known prion diseases attack the brain. Mad cow disease, a prion disease, results in the following symptoms:

Cows affected by BSE are usually apart from the herd and will show progressively deteriorating behavioural and neurological signs. One notable sign is an increase in aggression. Cattle will react excessively to noise or touch and will slowly become ataxic.

Deteriorating behavioral and neurological signs, increase in aggression, reacting to noise, and becoming ataxic are all exhibited by zombies.

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Welcome to the site pacoverflow. – James Jan 14 at 19:14
    
To all potential answerers: if you don't specifically discuss prions, then you should probably answer this question instead. – curiousdannii Jan 15 at 3:05
    
Here's the definitive paper: The Etiology of Romero-Fulci Disease: The Case for Prions – JDługosz Jan 15 at 4:40
    
Kuru disease is a prion based disease, spread from human to human by cannibalism of brains. en.m.wikipedia.org/wiki/Kuru_(disease) – Andrew Grimm Jan 15 at 13:09
up vote 1 down vote accepted

Here's the definitive paper: The Etiology of Romero-Fulci Disease: The Case for Prions

In particular,

Besides the evident histological similarities, there are some striking parallels between the modes of transmission and symptomology of TSEs and those of Romero-Fulci disease. Here perhaps the most compelling example is kuru, an endogenous disorder unique to the Fore-speaking peoples of the eastern highlands of Papua New Guinea. Kuru, which means “trembling with fear” in the Fore tongue, reached epidemic proportions in Papua New Guinea prior to 1971, when the practice of ritual funerary endocannibalism responsible for its transmission was abolished by law. Traditionally among the Fore people the cooking and consumption of the corpse—particularly the brain—of a recently-deceased loved one was considered a gesture of respect for the departed and an integral part of the mourning process. Because kuru’s incubation period can be as long as three decades, some older Fore who participated in cannibalistic rites are still dying of the disease. However, no young person has exhibited symptoms of kuru since the practice of endocannibalism was discontinued (14). Although the incubation period of RFD is radically shorter, the similarities between it and kuru are difficult to ignore: Both are fatal neurodegenerative diseases transmitted orally by an act of cannibalism focused particularly upon the brain matter. Kuru is known to be caused by prions; it therefore seems not unreasonable to propose—especially in light of the striking histological similarities exhibited in Figure 1 and Figure 2—that the causal agent in RFD is also a prion, albeit of a hitherto unknown fast-acting variety.

The main problem is that the speed of infection (or activation?) Is orders of magnitude more rapid than would be expected. The article speculates

The prion hypothesis is a strong contender, either as an alternative first cause or as a cofactor.

There may be a complex mixture of agents involved including other biological agents, poisons, and ionizing radiation. Prions are involved and behind the zombie effect, but not the sole cause. Even if prion conversion is the actual mechanism for the zombie state, other mechanisms are needed to disperse them through the tissue and catalize the needed changes.


Here's my remix: nanites are invented to fix tissue. They are to the point of mostly working. But unexpected prion foldings act like a bug in the system, just as they do naturally: the nanites try to "fix" things using the misfold as the exemplar! Meanwhile the body is kept working by any means the nanites can manage without caring that the brain is messed up. Rather than a persistent vegitative state though the misfolding affects the nanites themselves, making them (further) malfunction and create a Romero-style zombie.

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Classical "Zombies" - undead being which feed on flesh, and can only be killed by destroying the brain - cannot exist.

Basic biology forbids this.

Muscles

When you die the normal processes which power your body end. Your heart stomps pumping blood. You stop breathing. Without blood distributing energy and oxygen to your muscles, they can't function. If they can't function, you can't move.

It doesn't matter if a Zombie eats brains, or anything else. If that food is not broken down, and the energy not transported to the muscles then you can't operate the body. The whole system needs to be functioning, not just convenient bits and pieces.

The Brain

Within minutes of the brain being deprived of oxygen our neural pathways begin to deteriorate. There is a very real risk that if you reanimate someone after they were deprived of oxygen for a long time they may suffer from severe and completely unforeseen brain injuries.

Maybe they will have lost their memories, maybe they will be "alive", but never wake up from the coma they're in, etc.

The brain also "runs the whole show" as far as operating the body goes. There is no way that a dead body could rise after months of undergoing decomposition. Even after days of decomposition. There would be nothing left to "think" about eating or anything else.

More Likely

Rabies

A virus which makes people go crazy and attack others is not that far fetched. Watch the movie "28 days later" for a pretty chilling view of what could easily happen if the right animal activist breaks into the wrong lab.

These are not zombies, however, in that they will die of the same things we do: malnutrition, injury, poison, disease, viruses and bacteria - even the common cold could do them in.

A bullet to the chest would be just as effective as one to the brain (in the grand scheme of things).

They would still need to go to the bathroom, except they wouldn't remember how to take their trousers off, or how to wipe their asses. How long would it take for some severe and utterly nasty infections to set in?

Nano-Plague

If you're dead set on setting the undead loose on the world, then nanotechnology might level the playing field for you.

Nanobots wielding the bodies of dead humans and animals in order to fight their creators (us), could sound somewhat plausible in a sci-fi setting. I say this because you can claim that rather than having the body move under the power of its own muscles, nanobots would actually be facilitating the movement, not living cells.

All you'd have to do is explain why those nanobots did not simply choke the life from every human in the first place, or why they don't build robots to murder us all in our sleep, rather than shambling about in rotting corpses.

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I think I am Legend also went with the rabies-like infection. It was never fully explained what was going on, but the zombies weren't actually dead. – Draco18s Jan 14 at 19:30
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@Draco18s - I Am Legend is utterly ridiculous in that a "cure for cancer" goes wrong, infects mankind, and causes the victims to mutate into basically completely different beings. That's proposterous, because you can't just flip a switch and change every cell in a human being's body. Those "monsters" can be seen leaping great heights, climbing walls and walking on ceilings like Spider-man, etc. They were meant to be terrifying, but no explanation was given as to how an "infected" human might gain such ridiculous strength, grip, or what reinforced their bones to survive those stunts. – AndreiROM Jan 14 at 19:33
    
Oh I completely agree that a "cure for cancer that goes horribly wrong" doesn't make any sense. I only mentioned it as another example of a rabies-like infection. – Draco18s Jan 14 at 19:35
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John Ringo also went with a modified rabies virus to destroy higher brain function in his Under a Graveyard Sky books. His "zombies" are just hyperviolent and braindamaged to no longer being sentient but still living people. The person who created and initially spread the disease is never identified, but "Juniors Home RNA Kit" leading to disaster because people will do incredibly stupid things if given the ability has been one of his hobby horses for the last half dozen years or so. – Dan Neely Jan 14 at 20:48
    
@DanNeely - I've read the series. I think his "zombies" are a little too capable of surviving considering they are stark naked human beings with little to no intelligence, however. – AndreiROM Jan 14 at 20:58

There's a fungus that turns insects into zombies. I don't know how viable that is for larger animals, like us fleshy humans, but it does exist. I have a vague recollection that a TV show did this once for people as well (as Monster of the Week), but I can't think of what series.

I also happen to be reading EX-Heros right now and the vector in that book is a virus that as it turns out (spoiler!) was created on accident by a superhero with regenerative powers trying to resurrect his dead wife. So the virus being semi-magical having the described behaviour is plausible within the setting.

Edit:

Now that I think about it, Invasion of the Body Snatchers could be seen as zombie-like: the "infected" are actually aliens and will abduct people, make a copy of them (that's actually an alien) and reinsert them into the population. With the right behaviours and attributes, these aliens could be considered zombies.

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The book The Girl with All The Gifts uses that fungus as it's zombie vector. Pretty good story. Also the Ex-Heroes series is really good! His other series is also worth reading. – AndyD273 Jan 14 at 19:19
    
@AndyD273 I'm really enjoying Ex-Heros (via audio book, which is read by two voice actors which is uncommon), I'll probably check out the author's other books, but I've got to make a detour back to The Starflier War after I finish this series. – Draco18s Jan 14 at 19:28
    
Mmm, The Commonwealth series. Those 5 books were the best solid month of listening in a long while. I liked the narrators for the EX books too. It seems that we like a lot of the same books :) – AndyD273 Jan 14 at 19:39
    
@AndyD273 It seems that way. I'd have started a chat to continue this conversation, but I have no bloody idea how. – Draco18s Jan 14 at 19:43
    
@Draco18s just go to chat, create no room, and invite him. chat can be found by clicking the button where it has a drop down menu for stack exchange – Dragonrage Jan 14 at 23:19

I read a book a while by Brandon Mull, and in the series Beyonders, they have a zombie plague. It is cause by little worms with hive minds. These worms knit together wounds, and somehow work together to move the body, like a second set of muscles. They feed off of the blood of their victims, and take their bodies.

I find this scenario the most realistic, otherwise, how would a zombie stay in one piece, or get anywhere at all? Why would it feed on flesh, like in the walking dead? The food is irrelevant to their survival, and they don't think or breathe, so their brain would be dead and they'd have no reason to go eat people. But man, do these nasty little worms crave blood.

I don't think prions are realistic, because once the body becomes significantly damaged, then you have brain death, which mains you cannot move. Besides, what purpose would prions have with eating people? Unless these prions somehow supplied the brain with oxygen, and made sure the body was not damaged, they wouldn't be very intimidating. Break its leg and it can only drag itself very slowly, or crawl.

I recommend using some kind of parasite, because then there is an actual purpose to the madness of their violence. Also, some people might have a smidgeon of reason, maybe enough to barely keep a hold on themselves, or enough to strategize how to catch people. This might make your zombie apocalypse more dangerous and interesting.

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Seeing as you don't discuss prions, this answer really belongs on a question more like this one. – curiousdannii Jan 15 at 3:04
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I added a paragraph to make it better suit the issue of die ons. – XandarTheZenon Jan 15 at 3:12

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