How can I set up the environment so that my creatures don't get sick?
In a world I am working on, I have a wilderness-living, social, group-living species that for various reasons lacks access and ability to anything resembling modern medicine, including vaccinations. Yet, I want to keep it such that absent physical injuries, adults of this species very rarely become visibly sick or infirm, even when affected by what would ordinarily be relatively serious illness. The world they exist in is very much Earth-like, and might very well be Earth, but is not guaranteed to be Earth specifically.
These creatures are biological beings that evolved according to the theory of evolution as currently scientifically understood, with no superpowers, magic or "intelligent design" involved (except to the extent that I have an idea in mind and am working backwards to figure out how they might have evolved that way and if that is possible). I haven't yet decided on the exact form for this, but they have largely human-level intelligence.
How can I arrange their environment such that adults generally do not become sick, and when they do, that the effect on the individual is generally limited such that they basically remain able to function at a level similar to that of healthy individuals while their bodies are fighting off the disease?
It's perfectly acceptable and perhaps even good if some medical conditions cause a significant reduction in ability to function in some individuals (particularly very young and very old), but I want adults to for the most part be able to simply shrug off most illness.
Please don't just say "evolutionary pressure"; that's a given. Rather, be specific as to what could give the effect described. Bonus points for references and real-world examples, but any well-reasoned answer that would not break suspension of disbelief is welcome.
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