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Humans have landed on a planet that to the untrained eye appears very Earth-like. What small, at first unnoticed, differences in the environment would eventually have a negative impact on the humans and lead to moderate to severe sickness?

My goal is to have problems caused by a more intense sun, be it closer or slightly stronger than Earth's sun.

Are there illnesses/symptoms similar to heat stroke that heat stroke could be mistaken for.

It could be caused by unique pathogens, different levels of oxygen in the air, slight differences in gravitational pressure. Keep in mind these differences should not be immediately noticeable.

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  • $\begingroup$ What kind of negative impact are you looking to cause? Dead, sickness, euphoria, sadness...? $\endgroup$
    – Tridam
    May 22, 2018 at 19:33
  • $\begingroup$ physical sickness that is moderate to severe, but not leading to immediate death. $\endgroup$
    – user40179
    May 22, 2018 at 19:35
  • $\begingroup$ That's an awfully broad question, with a huge number of potential answers. Can you provide more detail on what you're looking for to help narrow down the possibilities? $\endgroup$
    – Gene
    May 22, 2018 at 19:35
  • $\begingroup$ I already know that I want the actual cause of sickness to be a more intense sun, resulting in sun stroke when the humans are fully exposed to the sun. But I would like more options for them to consider as possible causes of the physical weakness. As speculative causes, not actual ones, they don't have to be completely accurate, but just accurate enough to stress the colonists out. $\endgroup$
    – user40179
    May 22, 2018 at 19:39
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    $\begingroup$ Too broad, I could write you a book about the gases that could cause those effects, let alone anything else. $\endgroup$
    – Ash
    May 22, 2018 at 19:44

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Diseases are the first thing that comes to my mind. Many diseases have an incubation period with no symptoms whatsoever. Missing nutrients would take a while to notice, too. There are many things that require attention when trying to get started on an alien world. People are unlikely to take the time to carefully analyze the makeup of every food source they discover beyond verifying that they're not poisonous, especially after several "It's just like on Earth!" moments to make them complacent. Certain fatty acids are already in poor supply in many American diets here on earth. If they were non existent on your planet, it would take a while to notice, but eventually they'd develop deficiencies with noticeable symptoms.

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Taking inspiration from a Star Trek TNG episode (Nigth Terrors) you can have some pathogen that in some way (maybe as a collateral effect) induce the "Fatal familial insomnia", a disease that cause a person not to be able to sleep and a rapid collapse both physically and mentally and therefore to the inevitable death.

Normally is a genetic disease, but with a stretch of imagination it is possible make it as the effect of an alien pathogen on the human body

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Its not just the amount of oxygen in the atmosphere, but what else is in the air they breathe. Excessive carbon monoxide. Other chemicals that slowly mix with the water in lungs, creating toxic or acidic/basic substances. There are other naturally occurring substances like asbestos that can be in the air that can cause irritation, inflammation, and other respiratory issues. Plant spores, viruses, and bacteria can be deadly to humans as well. Its not just particulates in the air or microorganisms. It could be local insects or "poisonous" plants (like poison ivy). Small bites or chemical exposure could allow a substance that is safe to local flora and fauna that will slowly kill beings that didnt evolve an immunity.

The reality is, no intelligent being will ever willingly breath another planet's "Earth-like" atmosphere, or expose their bodies in any way, without exhaustive testing, over an extensive period of time. There are way to many unknown factors to make it safe. This is why people laughed at the War of the Worlds movie. Would an alien race capable of interstellar travel and associated levels of technology be so dumb as to come back to a planet they seeded with life, to die a few days later from microorganisms? Or, just as bad, the movie Signs. Aliens whose flesh is dissolved by water, comes to a planet that is 80% water, has water vapor in the air, and the atmosphere is known to RAIN down the substance that kills them. Doesnt make much sense.

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  • $\begingroup$ Is an infection by a bacteria/virus from another planet scientifically plausible? Even here on Earth, where all animals have a lot in common, we usually contract diseases from our own species. Can a human contract something like a cheetah fever? A turtle plague? A frog gonorrhea? There are things like malaria, of course, but that's nothing like human and mosquito suffer from the same disease - the microbes just co-evolved with us. Now, no alien virus would have co-evolved with us... $\endgroup$
    – Headcrab
    May 23, 2018 at 4:18
  • $\begingroup$ What?! Your statement is patently false. Why wouldnt it be plausible? Humans contract diseases from other animals quite often. This doesnt include animal to animal diseases, which is also common. Just google it, there are hundreds of diseases which can transmit from animals to humans. $\endgroup$
    – Keltari
    May 23, 2018 at 4:24
  • $\begingroup$ My statement wasn't that humans can't contract diseases from other animals (don't even need to google to debunk that - who doesn't know about stuff like rabies or anthrax?) What I'm saying, the closer two species are, the more common diseases they have. Take that stuff from your list, for example - it's mostly about other mammals, with occasional birds, and there are only two entries involving cold-blooded vertebrates (salmonella and fish tank granuloma). And an alien species wouldn't be just farther away - they wouldn't even share the same evolutionary ladder with us. $\endgroup$
    – Headcrab
    May 23, 2018 at 5:18
  • $\begingroup$ @Headcrab "they wouldn't even share the same evolutionary ladder with us" How do you know that? If this is an Earth-like planet, than odds are likely they will share a very similar evolution. Even on Earth, animals of completely different species and genus adapt similar traits. $\endgroup$
    – Keltari
    May 23, 2018 at 5:20
  • $\begingroup$ Yeah, sharks and dolphins look kinda similar, even though the former are fish and the latter descended from ungulates, but does it make them similar internally? Genetically? Share same diseases? I don't know much about biology (hence my original plausibility question), but when I occasionally read about viruses, it typically goes like "this virus uses this molecule to attach to that specific protein, and some people have that protein slightly different, so they're immune". So isn't it like infecting an alien computer with our Windows XP virus? $\endgroup$
    – Headcrab
    May 23, 2018 at 5:41