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Space explorers with same tech level as in alien franchise found a planet. Planet had some carbon based, earth-like flora and fauna on it, but it still needed some additional touches, not a full blown terraforming.

After ~100 years process successfuly ended, automated terraforming stations and bases stopped. Terraforming changed the planet ecosystem drastically, many species died off but others adapted and took niches, life on this planet is way more genetically homogenous so they evolve very fast compared to earth life.

Crew woke up from cryo-sleep and is ready to land, orbital station departed for other mission indefinitely. They took off suits but found out that planet (maybe atmosphere, maybe fauna, your call) is lethal (immediately or not, your call again) for everyone roughly over 25 terran years old.

Why ?

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    $\begingroup$ The only real-life phenomenon that I can think of that kills only certain age groups are pathogens (i.e. virus, bacteria, or fungus diseases). And even then, it just makes certain age groups more or less likely to die. $\endgroup$ – cowlinator Sep 14 at 21:27
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    $\begingroup$ How sharp do you want this cut-off for 25 years old? Any realistic toxin or disease would produce a "curve" with some people dying younger, and some staying alive for longer. $\endgroup$ – Alexander Sep 14 at 22:25
  • $\begingroup$ There is a Star Trek episode called "Miri" where everyone on Earth has been infected by a virus that kills people once they hit puberty. This episode, which actually takes place in Mayberry, the setting of the Andy Griffith show, would be a good place to start. The virus in question was man-made. $\endgroup$ – Shrike Sep 14 at 22:26
  • $\begingroup$ Terraforming for only a hundred years is unlikely to have triggered much in the way of evolution yet. It will take a lot longer to evolve new species, etc. $\endgroup$ – DWKraus Sep 15 at 0:05
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    $\begingroup$ Please explain what you mean by "alien franchise". $\endgroup$ – jamesqf Sep 15 at 5:04
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The oldsters had all been vaccinated. The youngsters not.

In the real world, I got the smallpox vaccine when I was a kid. My younger brother did not. They stopped giving it to kids in the US between the years we were born.

In your world the old folks all got a vaccine that came out of use or was modified 25 yeas ago. Younger persons got a different version or were not vaccinated at all against that pathogen.

The immune response induced by this old vaccine combined with a germ on this planet combine to produce an overwhelming immune response, lung edema and circulatory collapse. If the vaccine in question were smallpox, I would die on this planet but my brother would not.

He might sneeze a lot though. He does that anyway.

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    $\begingroup$ Or the other way around - here vaccines are mandatory for certain age school children, and the policy changes over time. In the last 25 years they started doing a vaccine, and that vaccine wouldve stopped or greatly reduced an illness. $\endgroup$ – Ash Sep 15 at 1:38
  • $\begingroup$ Yeh ditto on the smallpox, I was talking to my 22yr old son the other day, about vaccinations in a Covid age, when I noticed he didn't have the little scar on his shoulder, $\endgroup$ – BobT Sep 15 at 10:03
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Growth hormones & liver function:

A native life form is partly compatible with humans and is parasitic in native species. In humans, growth hormones are suppressing these things, but only kind of. Thousands of parasitic cysts start implanting themselves in people from the day they arrive.

These produce toxins, especially when they hatch. Liver enzymes are high in children and teens, but as you transition to adulthood, the enzyme levels in the mid-twenties protecting people from hangovers when young begin dropping off. Simultaneously, the last vestiges of puberty begin drying up, which allows more and more of those cysts you've been acquiring all your life to hatch. Exposure to toxins does tend to increase the production of enzymes by the liver, so "natives" might have more enzymes, but also more cysts. Chronic exposure to these toxins could be causing premature cirrhosis and liver damage, so your citizens might be already seriously ill before they reach a crisis. Youth and good regeneration are just covering up and compensating for the symptoms.

Now your body is flooded with dying toxic parasites and you are poisoned while your immune system goes into overtime fighting parasites and you die from violent chills, fevers, aching, and uncontrolled diarrhea, all while shedding parasite eggs in great gobs.

  • PS If the goal in this was to say terraforming was the cause of the mortality, then we can specify the parasite/s normal host, the neo-hare, Is globally distributed, but it's chief predator, the pseudo-fox, was wiped out. Now the neo-hare population has exploded, spreading what were rare parasites everywhere. A lot of treatments for parasites are often themselves toxic, so even if a treatment were available, it could cause liver damage itself, and killing parasites often causes them to release their toxins and triggers an immune response. Your people may be too infected to safely treat. People in space could visit the planet and get treated reasonably safely due to low levels of infection.
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  • $\begingroup$ Would not that kill women at an earlier age than men? $\endgroup$ – Otkin Sep 15 at 6:44
  • $\begingroup$ @Otkin Good point. I was a little concerned about that. It is a hypothetical, and we are creating parasites out of whole cloth, so odd things happen with diseases and biology. If the goal was to preserve the population, we could simply say estrogen suppresses parasites partly, even at low levels. That way, if women live to 26-28, there are more child-bearing years. Males don't matter in this scenario. $\endgroup$ – DWKraus Sep 15 at 21:10
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No, but you can create a similar effect

Reduction in Lifespan

As cowlinator said, there aren't any natural phenomena that kill every person in a certain age group. You could argue that certain diseases might be able to do so, but it would be unreliable. However, we can vastly reduce the average lifespan of your colonists so that living past twenty-five is incredibly unlikely.

Cause of Death

There are several directions you could take in reducing the average lifespan of a human, but one which I feel holds a lot of potentials is forcing every colonist to develop pulmonary fibrosis. This is not only consistently fatal unlike disease or natural disasters, but is currently incurable and probably still would be for your hypothetical colonists.

If the atmosphere of the planet was sufficiently saturated with a fibrous particulate, the development of progressively worse pulmonary fibrosis would be inevitable. You wouldn't be able to avoid exposure even with future-tech filters. [1] There are probably ways to industrially cleanse the air, but it's unlikely your colonists would have the necessary resources to do so.

Explanation

The easiest explanation for the presence of these particles would be the byproduct of a biological process. More specifically, the byproduct of aeroplankton. Without delving too deep into speculative biology, you could say an event similar to the Great Oxidation Event occurred after this specific aeroplankton was rendered hyper-viable by the tampering your colonists did to the biosphere.

I tried to find an example organic particulate to model yours after, but there weren't any low hanging fruit. So, just think things like silica, asbestos, and smoke.

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    $\begingroup$ Wow, this is a well thought out and terrifying answer. $\endgroup$ – cowlinator Sep 14 at 23:26
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    $\begingroup$ It's hard to believe that a filtration system capable of maintaining an atmosphere on board a self-contained space ship would be incapable of dealing with airborne pollutants of whatever variety capable of causing fibrosis... and, for that matter, detecting them before anyone yanks their helmet off. $\endgroup$ – jdunlop Sep 15 at 1:33
  • $\begingroup$ @jdunlop Most filtration systems for use in things that aren't wastewater treatment plants can make the assumption that the PPM of whatever your filtering is already low and that there is clean air somewhere. If you only have heavily contaminated air it won't matter how good the filtration is. $\endgroup$ – Evelyn Shepard Sep 15 at 2:27
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Youth can sense danger better

Few variations on a theme here:

  • A large cat like animal hunts in packs and uses high frequency sounds to synchronise their slow encirclment. The youth can hear it, feel uncomfortable, and slowly move away from the area, but adults cant hear it and can get eaten.
  • A tree that puts out a high frequency sound, and asbestos like fibres. Youth can hear it and avoid the area, adults enjoy the peace and quite and shade, and then die of asbestosis.
  • A mosquito borne plaque. Yellow fever. Ross river virus. Something like that. The mosquitos can be heard and slapped by the youth, but adults cant hear them.

Also it's not unheard of (at least here in Australia) to have animals that only attack a particular demographic. We had a pet gallah that would only attack adult males, my high school had magpies that would swoop and attack only teenage females.

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    $\begingroup$ Do drop bears favour older people, at least we are looking out for them, not like the ignorant younger generation! $\endgroup$ – BobT Sep 15 at 10:06

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