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Essentially, what I want to have answered is whether an alien ecosystem could exist without autoimmune diseases such as cancer, arthritis, and others (I don't know of any others off the top of my head).

Essentially, could an ecosystem be able to survive without too many diseases, maybe parasites and other, non-autoimmune diseases or infections.

And if an ecosystem can survive without autoimmune disorders, or many diseases overall, how would it be done? Like, for example, maybe it's by super strong immune systems that managed to evolve before many diseases could evolve?

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    $\begingroup$ Cancer is caused by unfortunate cellular mutations, so if your aliens’ cells somehow don’t mutate / are prevented from mutating, there won’t be any cancer. Not sure what to do about autoimmune infections though. $\endgroup$
    – ten million crabs
    Commented Oct 16 at 23:33
  • $\begingroup$ For future reference, you're allowed one and only one question, no matter how related they may be. Asking more than one question is specifically a reason to close questions (click "Close" and read Needs More Focus). Thanks. $\endgroup$
    – JBH
    Commented Oct 16 at 23:41
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    $\begingroup$ "maybe it's by super strong immune systems that managed to evolve before many diseases could evolve" that is not how evolution works. Why would an organism with a strong immune system (which takes a lot of energy) be more likely to procreate than an organism with a weaker immune system, given that the diseases, for which it is relevant, do not exist yet. It would be a it like humans living far from water to evolve gills 10000 years BEFORE their homeland atlantis sinks to the bottom of the ocean. Also "strong" immune systems with nothing to do are the leading CAUSE of autoimmune diseases. $\endgroup$
    – datacube
    Commented Oct 17 at 6:48
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    $\begingroup$ @tenmillioncrabs, without mutation, how would their evolution work? $\endgroup$
    – Going Durden
    Commented Oct 17 at 9:15
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    $\begingroup$ Incidentally cancer isn't an autoimmune disease. It is actually the opposite; where the immune system was supposed to kill malfunctioning (cancerous) cells but didnt. Autoimmune diseases are where the immune system mistakenly attacks healthy cells $\endgroup$
    – Richard Tingle
    Commented Oct 18 at 7:18

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Yes, whyever not?

The aliens of Aldebaraan II recently asked a similar question, “is it possible to have an ecosystem without tricellular self-detonating parasites that turn us inside out?”

There was much debate about this, as said tricellular organisms were so deeply essential to the Aldebaraan biological map that it was hard to imagine a biology without them!

And yet, here we are, just 65 light years away, all right-side in but immunocompromised!

Tl;dr: given that from a hard science point of view, we have an N of 1 for biology, it’s hard to say what else is possible or impossible. Everything that exists, does. Everything that doesn’t, doesn’t. So if you’re worldbuilding, decide what does and what doesn’t and go forth.

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    $\begingroup$ Hallelujah..... $\endgroup$
    – JBH
    Commented Oct 16 at 23:36
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Could an ecosystem exist without Autoimmune illnesses? Yes.

How could it be done?

Your world would need to have immune systems that evolved several key mechanisms to enhance its ability to distinguish between self and non-self while maintaining its effectiveness against pathogens/parasites.

  • You need a biological mechanism to "kill" any antigen that would trigger on the self. "An Immune-system for the immune-system." with some kind of regulatory cells that can "kill" or "deactivate" the cells that would go wrong.

  • Increase the amount of biological triggers required for an immune system response. This would likely allow infections and pathogens to progress much further (then what we experience) before a reaction occurs, allowing false positives to be avoided.

(Note, this will make infections of any type significantly more lethal due to the slower response time)

  • Some kind of Cytokine regulation system, that would allow you to avoid being "too inflamed" or "non-responsive"

  • Symbiotic Methods, the concept of a gut microbiome could be expanded into the body. (Combine this with higher amount of triggers required for a response to ensure they are not wiped out). It is possible your ecosystem has many "germs" that are symbiotic and act as a part of an enhanced immune system for a particular organ or system.

(Perhaps this symbiotic germs are the only thing that prevents infections from harmful things getting to "rooted" into the body, mitigating deaths)

Or go entirely the other direction.

  • The ecosystem might have a very low level of harmful substances, a more "paradise" world type situation. In which case... "what immune system?"
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    $\begingroup$ I'm imagining an ecosystem comprised exclusively of different varieties of slimes that quickly and efficiently digest anything that penetrates their outer membrane into its constituent compounds for nutrients. To such an organism, there is no need for an immune system because the digestive system effectively is the immune system. $\endgroup$
    – Abion47
    Commented Oct 17 at 22:10
  • $\begingroup$ @Abion47 How do the slimes remain... separate? $\endgroup$
    – Wyrsa
    Commented Oct 18 at 7:27
  • $\begingroup$ As an evolutionary measure of avoiding the consequences square-cube law, slimes regularly shed excess mass. And due to the slime cores being resistant to being digested, devoured slime cores often end up getting shed in the process as well, resulting in the shed mass effectively becoming a new slime. $\endgroup$
    – Abion47
    Commented Oct 18 at 15:16

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