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My original idea is:

A group of scientists decides to kill all adult people (say people over 18 years, include themselves) and leave the world for children/youth, because the children are not corrupted yet. They want to discover a way when all adults will die in one exclusive event and don't changes anything else, that means, all the survivors can grow up normally until the adulthood and then fix the problems created previously by adults.

The initial plot is pretty much like the initial plot of the comic book "Y:The last Man", but with adult people rather than men.

So, my question is: What's the best scientific explanation to kill all the adults and preserve the children/youth? There are some disease or weapon (biological/chemical weapon, I mean) that just affects adults in the real world?

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closed as off-topic by Pavel Janicek, JDSweetBeat, bowlturner, Serban Tanasa, Mason Wheeler Sep 4 '15 at 20:51

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If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I feel like there should already be novels, comics, movies or TV shows based on the same premise. I don't have time to do research now, but you should find inspiration in these. $\endgroup$ – Babika Babaka Sep 4 '15 at 14:49
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    $\begingroup$ There is a star trek episode Miri en.wikipedia.org/wiki/Miri_(Star_Trek:_The_Original_Series) $\endgroup$ – bowlturner Sep 4 '15 at 14:57
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    $\begingroup$ This would have seemed so much cooler before I was an adult. $\endgroup$ – corsiKa Sep 4 '15 at 16:38
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    $\begingroup$ See: tvtropes.org/pmwiki/pmwiki.php/Main/OnlyFatalToAdults $\endgroup$ – A E Sep 4 '15 at 17:17
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    $\begingroup$ How does the Dedekind cut need to be issued? Must it be based on precisely how many times you have hurtled around our local great ball of fusion fire in the sky? Biological clocks tend to be more distributed, so the "biological clock" of each cell is going to be slightly different. Can the line be drawn differently such as "those who have been through puberty," or "those who have learned a particular bit of information that leads them to do something fatal?" or "whatever the homicidal maniacs with guns think look over 18?" $\endgroup$ – Cort Ammon Sep 4 '15 at 18:36
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A virus that cuts off Telomeres that are too short.

To quote Wikipedia: "A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. [...] For vertebrates, the sequence of nucleotides in telomeres is TTAGGG. This sequence of TTAGGG is repeated approximately 2,500 times in humans."

Very roughly speaking: when cells replicate, the telomeres are shortened. This means that in young organisms, telomeres are long, and in older ones they are shorter.

So... assume a virus - like a common flu virus - that has been mutated and targets cells with too short telomeres and simply cuts them off. Cell replication will be shot to hell and your chromosomes in each cell afflicted by the virus will be a jumble; the cells die. Once it hits it will probably be like a severe hemorrhagic fever (like Ebola or Lassa). If your telomers are long enough, you are not afflicted at all, your body adopts normal immunity to the virus and defeats it.

Weaknesses in this: individual variation, individual immunity. There cannot be a hard limit that says "Until 18 years, 0 months, 0 days you'll be fine... above that - one day later - it is 100% mortality".

But allowing some fuzziness in the outcome - such as a few kids die and a few adults survive - this could perhaps be sufficiently credible. To increase the credibility and avoid nosey questions - especially since cells replicate at wildly different rates in the body - you can have the virus target a specific organ, like the brain (compare to the movie Contagion) or the heart.

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    $\begingroup$ Darn it. When I saw the question, this was going to be my answer. $\endgroup$ – Jake Sep 4 '15 at 15:48
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    $\begingroup$ This is a pretty gruesome way to die, if affecting the whole body it would pretty much have the same effect as something like the answer to the question "What if all your DNA suddenly vanished?" in Randal Munroe's book What If? whatif.xkcd.com/book (he likens it to severe radiation poisoning) $\endgroup$ – thanby Sep 4 '15 at 18:55
  • $\begingroup$ Yup.... that is what happens if your DNA gets broken. DNA is the "program" of the cells, that run everything in them. If that stops working... everything in the cell just break down. What radiation does is that it chops up your DNA. If it happens just a little - like with normal background radiation - the DNA repair mechanisms fix it easily. But when you get a massive dose.... then the repair cannot keep up and the DNA is damaged beyond repair, and the cell dies. $\endgroup$ – MichaelK Sep 4 '15 at 20:07
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You can imagine a biological weapon that reacts to hormones.

For example, you could use a virus/bacteria that reproduces only in the presence of high levels of sexual hormones (like testosterone and estrogen), and dies off in low-hormone environments. All adults and teenagers would die, but if the virus can't survive for too long in the bodies of children, it will then disappear in a few months.

Since I don't have much knowledge on these subjects, I don't know how a virus or bacteria could be influenced by human hormones, but I think it could be a good start.

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    $\begingroup$ One problem might be that the elderly would linger for a while... I think this is feasible for a bacterium, if it had enzymes that metabolized the sexual hormones into toxins. If OP is okay with the deaths of some children, making it have "traditional symptoms" as well would probably decimate the elderly. I'm not sure how realistic a virus is by comparison, although it's certainly possible. $\endgroup$ – Nathan Ringo Sep 4 '15 at 19:19
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Chickenpox is a good example of a real-world virus that could do what you're looking for.

Highly contagious, much more dangerous for adults than children, and comes back after extended dormancy for children who have had it in the form of shingles. Any similar virus or a mutation thereof could result in your desired outcome.

As an added plus, your audience may already be familiar with this mechanic.

Varicella-zoster is the virus that causes chickenpox in children. Adults who have had chickenpox as children are generally safe from being affected by it. Adults (or teens) who have not had chickenpox or a vaccination are susceptible to chickenpox and can have serious complications resulting from it, including pneumonia and encephalitis (swelling in the brain).

Adults who have had it as children may be safe, but they are highly likely develop shingles due to a reactivation of varicella-zoster:

Research begun in the 1950s has shown that when we recover from childhood chickenpox infections, the virus that causes the infection, varicella zoster virus, remains latent in nerve cells.

What causes reactivation of the virus is unclear, but as we age, experts believe the immune responses that keep varicella zoster virus dormant in the nerves weaken with age. One in three people will get shingles during their lifetime, and at least half of all people 85 and older have had the ailment.

Shingles can also cause encephalitis.

Tweak the numbers and increase the complications and you've got a lot of dead adults.

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First you have to find what makes childs and teenagers different from adult people.

I see at least 3 things that change while growing up :

1. Hormones

(as Spacelizard said - I can't add anything)

2. Organs and bones

The one and only idea I have about that, is a degenerative disease making organs and/or squeleton weaker and weaker with time, slowly killing everybody before a certain age. There are many existing diseases with similar effects, i think you can pick any part of the body and make it slowly crumble. But it will take time to kill your adults and will make your childs/teenagers weaker with age, so here is number 3 :

3. Brain.

The white matter in brain grows into childs brains until they are teenagers. We can imagine a disease in this matter becoming letal only when there is enough of it. It can be a virus, or a poison in air/water that accumulate in white matter.

Another one : a child maturity can be seen on a MRI. One of the last part of brain that changes during teenage years is the prefrontal cortex. Grey matter thickens in childs brain, then thins as the brain creates new and more efficient connections as a young adult. I'm far from an expert and the following ideas might be crazy :

  • A disease destroying brain when there are too frequent electric activity : unefficient childs brain with variable connections would be protected.
  • A sudden shockwave (that might be magnetic) on a frequence near of brains cells activity, adults with a thin grey matter are not protected enough whild childs just grow new connections (after a very bad time)
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Antagonic pleiotropy

Some genes benefit us in early life (ie. before we usually have children), and are detrimental afterwards. These genes proliferate because they increase the chance of the individual having children, even though they can wreak havoc afterwards.

So you could find one of these genes or invent one, something we all have, and then add a condition (virus, bacteria, radiation) that massively amplifies its effects. The young would get fitter, and the old would die off.

Tyrabel's hormone answer is a specific case of this, since a large amount of testosterone is apparently detrimental later in life.

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Disclaimer, I don't know enough about how diseases work to verify how possible this is so if this is wildly off base comment below and I'll either edit or remove it

Something that could work would be some sort of disease that spread around the globe that was largely benign. Also, for whatever reason while contagious enough to give the 100% saturation desired by the question, children born from people with the disease have immunity or resistance to it or its effects.

Now, fast forward 18 years and the disease which has previously been mostly dormant mutates or otherwise changes, becoming lethal. Everyone born over the last 18 years has inherited immunity/resistance due to their parents having the disease, however nobody over 18 would have gotten that chance. After that point, everyone alive is safe from the disease and life can continue for those 18 and under even as they grow past the threshold.

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There could be some type of "mysterious event" that subtly changes the bodies of everyone on the entire planet. (Maybe a burst of "cosmic rays" from the sun, or some type of explosion at a facility testing some fancy new technology...)

Unbeknownst to the Earth's population, it alters their bodies, making them susceptible to a certain type of ray/radiation/something.

18 years later, there is another "mysterious event", which releases the particular trigger that kills off everyone who was alive during the first event. Children who were not yet born/conceived at the first event are unaffected by the second.

I think this works better than a disease, because it's easier for it to affect everyone on the planet (especially if the particular ray/radiation/whatever can travel through all solid substances, and there's no way to shield yourself from it).

A disease will always have some people who are immune, or are isolated enough that they never come into contact with it. Also, with today's technology, governments would quickly take steps to quarantine the disease to keep it from spreading (look at the recent Ebola outbreak -- while still very devastating in the areas where it took hold, it was prevented from becoming a global pandemic because of strict measures taken by many governments to ensure it did not spread).

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