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In my world, there has been a zombie apocalypse at some point in the past. Humanity got hit hard by it, and the number of humans has been reduced to a few thousands when it happened. Humanity managed to actually win against the zombies over time, since there was a huge but limited number of them, and they are pretty dumb.

Now the human population is back within the millions (it's been decades, maybe even centuries, I haven't decided yet). We are rebuilding civilization slowly but surely.

Obviously, zombies didn't fight among themselves or eat each other (that would make any sort of horde impossible, therefore making them no dangerous at all). They aren't particularly interested in eating brains, but do love eating humans. They would probably eat other animals in they managed to grab them. Only humans can be turned into zombies.

They act basically like in The Walking Dead as for moving slowly, going after survivors by sound, sight and smell, but you need to have had contact with a zombie (bite, scratch, etc) to turn into one. You don't turn if you die from other causes, but your dead body can be turned later on if it gets scratched or bitten.

Now from a realistic perspective, even though they are technically dead already, zombies move. They need bones, muscles and energy to do so, even if they are almost brain dead, and don't seem to feel pain or bleed out from cut limbs. A human that gets turned probably has calories to keep moving for a few days, maybe weeks if he has lots of fat. Catching something and eating it will obviously result in a calorie intake (they can diggest you assuming they still have the necessary organs, and this can probably be handwaved if they don't).

Let's say that zombies show up on day 1, and reduce the world population through kills and turning them into zombies to a few thousands within 5 years. How long can I expect zombies to stick around after that, assuming humans are now well organized and almost nobody gets turned anymore? I'm looking for the very maximum length of time that there could still be a few zombies around, and what exactly did those outlier zombies do in order to last that long.

For example, if the majority of zombies died within 1 year, almost all of them are dead after 3 years, and maybe a few dozen of them can still be found after 5 years. What did those last ones do differently to last so much longer? Did they happen to be in a specific location, or were "asleep", etc. Where did they find the energy to live so much longer, and of course having an idea of how long this would be is good, but that's not the main question.

In this context, I want to set my story up so that everyone knows about the zombie apocalypse, but do not expect to find zombies anymore, but end up finding one or a few of them at some point, which take them completely by surprise (unexpected), but is still possible.

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closed as too broad by sphennings, L.Dutch, Aify, Azuaron, Mołot Jul 26 '17 at 18:05

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ It is extremely simple to calculate the daily calories needed for humans these days because of the amount of people trying to optimize their weight. I do not think it would be too much to ask that you could provide us with numbers how much you think your zombies need every day. If the metabolism isn't working as it used to, you can go down a bit with your resting metabolic rate for example - maybe end up at 1.2k kcal or something at the end, but that is really purely your decision. Please provide those numbers. If you do not know how to do it, ask a question about it in a different thread $\endgroup$ – Raditz_35 Jul 26 '17 at 11:42
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    $\begingroup$ You say they work just like in TWD but we know nothing about how the zombies work in TWD. Take TheLastOfUs for example, which uses a real life zombie fungus (cordyceps) except it moved from ants to humans, that is a very specific explanation of what the zombies are : "Humans controlled by a brain fungus". If your zombies are just dead bodies moving it makes it very difficult to hard science cuz they make no sense whatsoever. Let alone sustenance, they can't move without blood and with rotten muscle tissues. $\endgroup$ – Liquid Same Jul 26 '17 at 11:44
  • $\begingroup$ @Raditz_35 I might have been unclear. I don't know how much calories they need, and that is part of the question. How much calories do you need to shamble around (and occasionally chase a survivor), groan and do extremely limited brain activity? I assume that's a lot lower than what we need for our day, since they are thinner and do much less movement. If they need only a few hundred calories, the answer would be a lot different than if they still need a thousand. $\endgroup$ – Kaito Kid Jul 26 '17 at 11:51
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    $\begingroup$ I'm afraid undead zombies can't really work at the level of scifi hardness (i.e. any) you require. Rage pathogen variety could, but those "zombies" are very much alive, with all the same weaknesses as healthy human. In addition, for rage-type, hunger induced infighting and cannibalism would be a major factor causing attrition of horde. $\endgroup$ – M i ech Jul 26 '17 at 11:53
  • $\begingroup$ @LiquidSame I was not talking about how they work, but specifically how they act. That was a very bad choice of work on my part (I'll edit it). I meant that they shamble, move, chase and eat mostly like in TWD. I can handwave anything for what caused the apocalypse, since it's not the most important part of the story (more like background info and maybe a chekov's gun or plot twist). $\endgroup$ – Kaito Kid Jul 26 '17 at 11:53
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So you want your zombies to be actual dead people. The only parallel I could think about are jellyfishes.

Jellyfishes are bloodless, heartless and brainless moving creatures. Close enough to a zombie for us to compare them.

Because your zombies lost their blood and therefore cannot carry oxygen and nutrients to the muscles for your zombies to walk, they won't be able to hunt anything and they won't even want to because their brain is dead as well.

The thing is, while jellyfish don’t have a brain or central nervous system, they do have a very basic set of nerves at the base of their tentacles. These nerves detect touch, temperature, salinity etc. and the jellyfish reflexively respond to these stimuli. Since they don’t have a brain, they live passively, depending entirely on their automatic reflexes.

First, we should understand exactly what a heart does. It basically pumps blood around the body so that the cells of an animal can get oxygen and nutrients from the blood and expel soluble wastes, like carbon dioxide. However, what is special about the jellyfish anatomy is the thickness of the outer layer, known as the Ectoderm. It is only a few cells thick, so oxygen simply diffuses into the jellyfish body. source

Zombies have holes everywhere so oxygen can travel directly to their muscles. Muscles are not a few cells thick so they'd lose a lot of muscle power. Don't expect them to do anything except crawling and faintly scratch someone. Oxygen won't just magically go into your muscle tissues so you'll need something to diffuse oxygen into your zombie which we'll get to later

Jellyfish also have a very rudimentary digestive system, where simple nutrients merely diffuse into the rest of the body.

When your zombie has no more energy, he'll start to crawl then just lay on the ground and go into hibernation mode. The idea would be to have your zombie bury himself in plants and dirt in order to pump the nutrients out of it. Maybe your zombies have a symbiotic parasite that pumps and processes nutrients for your zombie keeping it "moving" while the parasite uses the zombie to spread to other humans.

I told you about the cordyceps a fungus which invests an ant's brain and controls it towards leafs where the ant is to hang to and die until a branch pierces out of the ant's skull and releases spores. This fungus has a secondary metabolism which creates antibiotics for the ant to keep it from multiple infections while it hosts the cordyceps. source

Here are a few videos of the cordyceps' effects.

If instead of antibiotics the cordyceps were to produce glucose then you have a way to power your zombie pretty much indefinitely. The bigger problem comes from the state of decay in which the zombies are and would result in them falling apart within a few weeks of infection (depending on the weather). It would also mean that zombies would not survive inside buildings and cities outside of green spaces.

Your zombies will likely not look human anymore. Not even in the shape of a human because of the fungi growing off their bodies.

How the cordyceps went from the brain of a very specific species of ants to any human brain will have to be hand-waved because it is impossible (or would take millions of years of evolution). But this is the closest I can get you.

By working on senses and reflexes alone like a Jellyfish and with the help of a symbiotic parasite (likely a fungus) you could have something close enough to a "real zombie" which would grow roots to feed themselves in the absence of humans to eat


Edit: I jumped the "would that be enough energy part for now". Since you are very vague I do not want to go through all the energy conversion formulas before I even know if this is a fine concept for you. (And also I don't have enough time at this very moment. Will edit later if this satisfies you)

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I will offer what may seem like the "obvious" answer, in retrospect: Zombies burn their existing fat and are cannibalizing their own body. This includes cannibalizing their own brain (through biological processes, not grabbing pieces of it) which is mostly fat.

We do the same through exercise and starvation (and if we run out of fat, our body starts burning our muscle tissue, which is how we become emaciated).

I would guess one reason Zombies want brains is for their fat content; and perhaps Zombie digestive functions have been compromised to the point that it doesn't let them digest anything but fat.

This has some measure of plausibility; fat is already converted into an easy to access energy storage unit, that is why we get fat. So it is easy to break down. The brain is the fattiest organ in the human body; consisting of at least 60% fat.

An adult human, as food, offers about 100,000 calories. If Zombies need maybe 1000 calories a day (slow moving, little brain function, little healing or other bodily maintenance), their own body could sustain them for about 100 days. By then, other Zombie bodies will have been burning through their calories and offer little sustenance.

So once the regular humans run out; along with dogs, cats and cattle, give the Zombies four to eight months before they start dropping dead. (again, this time for real!) They drop dead when their metabolism, like a brain cancer, finally dissolves whatever neurons were left in their brain that was coordinating their movements.

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