A disease with an incubation time of 18 years kills all adults after its "timer" goes off. How can I keep it from affecting those under 18?
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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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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