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An airborne disease with an incubation time of 18 years goes unnoticed, but spreads across the world. 18 years later, its "timer" goes off, killing all adults. Good idea (thanks, @Morfium !), but I have some concerns.

Would it not spread to infect those under 18, thus killing them after another 18 years of incubation? I need my characters to be able to survive past 18, which is not possible if the disease affects a large majority of them. Could I use the logic that those born after the initial wave of the disease got natural immunity from their mothers, and therefore are affected but not killed when the disease is finished incubating?

To clarify, the story is about a society of children attempting to survive and rebuild after the death of all adults due to a worldwide plague.

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    $\begingroup$ Please, one question per question. $\endgroup$ – Mołot May 9 '17 at 16:00
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    $\begingroup$ This is not how diseases work. For one, there would be a huge amount of different strains in different people. Furthermore the idea that all people would have the same incubation period is impossible, people with weaker immune systems would succumb quicker. $\endgroup$ – Feyre May 9 '17 at 16:23
  • $\begingroup$ How about a disease that can only be contracted via direct injection? Simply inoculate the population in some sort of mandatory "for your own safety" sort of way and pick your target group. $\endgroup$ – James May 9 '17 at 17:16
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    $\begingroup$ So I will just say this. 100% coverage will in no situation be believable. Humans have a problem with complete certainty, and any situation you concoct to get their will naturally not be believable. Just a point I thought will be relevant. $\endgroup$ – James May 9 '17 at 17:36
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    $\begingroup$ So... Jeremiah, huh? $\endgroup$ – The Nate May 10 '17 at 15:51
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The exact mechanism can be left up in the air. It is not necessary for your story. You can have some disaster in a bioweapons facility. The scientists think - "no harm done". Then the badness comes home to roost 18 years later.

Leaving it not entirely fleshed out is a fine way to do it if your story is from the viewpoint of the kids after the fall. They would have no way to know exactly what went down. Theories from grownups might be in the news for a week or 2 but as civilization falls that will stop. I can envision a fine sequence where know-it-all nerd kids, instead of hoeing weeds, are arguing with each other about what happened and why. Some kid has memorized a list of things that happened just before the birthdate of the oldest known individual who did not die. They argue about each and how it possibly could have resulted in what happened.

Actually the kids working the fields and this discussion would be a fine way for the story to begin - the nerds obviously picking up an ongoing and longstanding argument. Dialogue is better than didactic.

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Based on your other question it doesn't necessarily need to be a disease correct? This was simply a great answer you got to that question.

What if it isn't a disease, but a hazardous substance everyone was exposed to 18 years ago which is having an effect 18 years later. Maybe an element from space entered the atmosphere, and was effectively recycled after a small amount of time on earth, but those who did come in contact with it were damaged in a way that eventually kill all of them in approximately 18 years? The children survive because they were not exposed to the hazardous material?

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Parasitic cicada-wasp-aphids

(This doesn't actually have to be as dramatic as infectious insects, unless you're into that kind of thing. It could be a tiny fluke or symbion that happens to have a life cycle similar to a cross between a cicada, a wasp, and an aphid.)

19 years ago, a new parasite was either created or discovered. It may be a mutation from a parasite that infected another species or the creation of a mad scientist or weapon project gone wrong.

In the adult phase (the original adults were either lab-grown or matured in their original host species), it is extremely infectious, but effectively harmless. It is capable of reproducing asexually through parthenogenesis, producing tiny adults that skip their larval phase and go straight to asexually-reproducing adults. It quickly spreads throughout the world, becoming a global pandemic.

It then produces males, which mate inside their hosts. The males die and the females burrow into the brain (or any other vital organ of your choice), lay eggs, and die. All of the adults die off after reproducing.

These eggs hatch into larvae. Like the 17-year cicada, the parasite has an extremely long larval phase. It is small enough to feed off of small amounts of blood without harming the victim. The larvae do not migrate, so any children born during this period are not infected.

18 years after the worldwide pandemic phase ended, the larvae mature. They then attempt to break out of their hosts, but due to a failure in their design are unable to break out of humans (unless you want this to be a cycle that recurs every 18 years). However, attempting to break out does kill the host. The only humans remaining are children born after the initial infection phase.

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As I said in ym comment to Feyres answer, make them realize they are infected and develop a medicine that only works on children that are not through with puberty.

That way the adults are sure to die, yet the cured children will survive. If you want less older children make it work better the younger the person that is cured. This might involve some handwaving though.

Another alternative, make the cure only work on pregnant woman. So the woman gets the cure, the baby in the womb is immunized, the mother dies eventually.

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A big thing here, is that a realistic and 100% idea will never mix. You cant have both in real life. That being said, if we want to get as close to 100% as possible, the best option is to figure out what the difference is between teenagers/children and adults that is not arbitrary like age.

One idea, Almost by definition, is puberty and development. Until we are around 21 years old on average, we are still developing. It may be slower than when we were 13, but its there. There is a reason why the drinking age for alcohol in the USA is 21. The brain is still developing throughout all teen years, and even into their 20s for many people.

So, if we tie this virus to the brain, with fully developed brains being helpless, we will kill almost all adults, but leave a few 20 year olds, as well as kill a a bunch of older teenagers.

The hard part is what specifically it does, and how a brain that is not done developing could survive, but fully developed ones can not. This is where we get into unknowns. One solid choice is that an undeveloped brain is more 'elastic', and so we could say that those brains can work around the virus. People can live without large parts of their brain. We could say that this virus affects critical functions, and a developed brain cant work around them fast enough, killing the adult. A child's brain on the other hand, with its elasticity, could theoretically work around the defects fast enough that they don't die. From there, the virus has killed off everyone it can, and those left behind are immune, and so it dies out forever, or keeps spreading to new born's, but cant kill them and so is harmless.

A realistic scenario here, the virus is rapidly spread over the course of a year by a malicious entity (perhaps fanatics who want to purge all humans). it takes 2-4 years to kill the person, but if your under the age of 16, you are likely to survive. After that, its harmless. You either die, or become immune to it, and no child under the age of 13 when infected will die from it, leaving the whole world dead or immune, with 99% of the population being under 18.

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Some food additive got banned 18 years ago.

It weakens the immune system and the common flu becomes deadly to those who had that supplement.

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    $\begingroup$ Actually, perhaps the opposite. A particular food additive that WAS in everything helped block the disease until it got banned. $\endgroup$ – Erin Thursby May 10 '17 at 3:54
  • $\begingroup$ @ErinThursby, Then why do the young people survive the disease? I was going for an effect that would not affect those under 18. $\endgroup$ – ShadoCat May 10 '17 at 17:37
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    $\begingroup$ Children are different than adults. Eating something, even over time and it still having an effect 18 years later on an immune level is odd indeed, and I know plenty of people that don't eat processed foods. If it was in everything the effect would be so devastating AT THE TIME it was ingested that there's no way the whole adult population would have been exposed to it. There are drugs that, taken once for some folks, will mess you up for the rest of your life, but you darn sure know it at the time. $\endgroup$ – Erin Thursby May 10 '17 at 18:04
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I'd say, kill some small percentage of the children, too, like 20%: Say any and all women that are pregnant when they are first exposed to the disease, pass it on to their fetus; but the immune system between the mother and the fetus is mediated by the placenta and other mechanisms (e.g. the bloodtype of the fetus can be different than that of its mother). So this filtering effect causes the fetus to "grow up" with the disease while its immune system is still "learning the ropes" (IRL immune systems must be trained by exposure; i.e. we "gain" immunity --- so if the fictional disease is one of the first things a new immune system ever sees, it crushes it), and because of that about 80% of the fetii exposed to the disease while in the womb just have immunity to the disease. The robustness of immune systems is not uniform, either; they vary from child to child, so for example some get sick more often, some less often. So in the story about 20% of children, born with the weakest immune systems, will spontaneously abort or die shortly after birth.

Since all the children conceived after the initial exposure will have been exposed in the womb from the start; nearly all of them survive. Presuming the disease vector is persistent, then in the subsequent generations, a newly conceived fetus will have the benefit of two immune parents, and be even more likely to survive.

Added: I don't wish to modify the above because it has already received votes, but as per comment by @TheNate below; if you find my biological explanation wanting; there is a simple bright line you can exploit: 18 years ago, everybody under 18 'now' was still in the womb or not yet conceived. That is the bright line.

I would add that for a short-lived pathogen, being in the womb may have protected them from exposure or infection; and that could apply even if they were conceived by infected parents; such persons still develop from a single cell inside the womb, and could plausibly be protected. So if the pathogen itself infects everybody but dies out in a matter of months; you have a bright line: Everybody born after it dies out was protected in the womb, and never gets infected.

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  • $\begingroup$ Selective immunity develops after birth, which is why mother-provided immunities via milk are so important. That leaves the logical problem that the mother must have developed this immunity to transmit it. Exposure to a pathogen without selective immunity is simply giving it more time to get established. $\endgroup$ – The Nate May 10 '17 at 15:58
  • $\begingroup$ @TheNate: I think you missed the point that this is fiction, specifically a form of science fiction, and certainly we don't know everything there is to know about how the immune system works (or nobody would die of immune system disorders). The OP is looking for a bright line between 18 YO and everybody else; one such delimiter is everybody older than 18 was out of the womb when infection began, and everybody under that age was still in the womb when first exposed. Thus this serves for the science fiction point of inflection, and kids don't have to explain exact details about why. $\endgroup$ – Amadeus-Reinstate-Monica May 10 '17 at 20:18
  • $\begingroup$ We don't know everything, but we do know that. Your explanation is the rough equivalent of saying "Well, he is smaller and weaker, so that's how he lifted what the others couldn't." If that doesn't bother the op, then all's well, but the biogeeks will still cringe. $\endgroup$ – The Nate May 11 '17 at 7:59
  • $\begingroup$ @TheNate: No, it is the equivalent of saying that in combination with the mother he has specialized immune system equipment that independent adults do not have. Science fiction does not have to reflect your assumptions about how things should work. Further, it is possible that because he is weak but supported by his Mother, that was the deciding factor. Or perhaps the pathogen does not affect organs, like the brain and lungs, that are developed in its presence. Or many other criterion unique to a fetus. You lack imagination. $\endgroup$ – Amadeus-Reinstate-Monica May 11 '17 at 19:03
  • $\begingroup$ The biology stack is a good place to seek out more details if you don't understand or believe me. If you removed the parts based on erroneous biology to focus on that "bright line" you mention, which is truly a reasonable dividing line, you'd significantly improve your answer. $\endgroup$ – The Nate May 11 '17 at 21:26
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Dengue fever is tough, but a secondary exposure to a different strain of Dengue causes hemorrhagic fever (see the wikipedia page under Mechanism > Severe Disease).

A disease that first swept the population (but was survivable) 18 years ago, followed 18 years later by a second wave of a slightly different strain, could cause hemorrhagic fever in people over 18 who had survived the first exposure.

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Although I can't think of anything that would be 100% effective at the age of 18, perhaps a hormone-based solution might do the trick?

The disease feeds/is stimulated by sex hormones such as estrogen and testosterone and produces a mildly toxic byproduct from the interaction.

This could be balanced in such a fashion that once they become "adults", the concentration of the toxic byproduct kills them.

The result would be a disease where no-one ever becomes a mature adult.

The drawback would be that it is not precisely 18 years, since human development speed varies, and that people with hormone deficiencies could survive the disease.

Nevertheless I hope this helps bring more ideas into the answer pool.

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