I was talking recently with a friend about illness - we both have kids, so we do this quite a lot. We were discussing how to "read" a cold, to tell when we should take one of the kids to the doctor rather than just waiting for them to get better. During this discussion I realized that though she and I were trying to do the same thing, we were using different metrics for how to decide when we needed to worry.
My friend's model of assessing whether to go to the doctor was based on the passage of time. A cold that needed attention was one that had been going on for a long time. My model of assessing whether to go to the doctor was based on trends of change. If the cold had been slowly improving and then appeared suddenly to get worse, I figured it was time to get attention.
I think if you were to ask around, you'd discover that almost everyone has a slightly different model of assessment that they're using. In fact, I'd be curious to discover whether medical practitioners are taught to use precisely the same models, and whether that results in them applying the same models and metaphors to what they see, or different ones.
One of my kids' books talks about early health beliefs. These are things like the belief in the existence of miasmas - evil drafts of air that carry disease - or an understanding of the body based on the influence of the liver, or the influence of the spleen, etc. I think we're all familiar with the idea that the heart is the source of love in the body, but I didn't know, for example, that some people believed that function was performed by the liver. Or that the gall bladder was seen by some as the seat of courage. Or that others believed that Saturn rules the right ear and Jupiter rules the feet.
Different levels of technology have some influence on body models, because they give people the ability to observe how the body operates. On the other hand, it's good to remember that people have an enormously strong tendency to create metaphors for the operation of aspects of life and the universe. Rulii in "Cold Words" used his hunt metaphors and had quite accurate knowledge of body parts and their operation (as a result of his hunting experience), but couldn't conceive of the idea that it would be possible to look into blood and see things.
The aliens we create can similarly have different models and metaphors for the body and its operation, and if you use those things to your advantage, they can influence characters' behavior and judgments, and possibly even the plot of a story. If a character were injured for example, why would or wouldn't he/she decide to get treatment? How would that influence the course of the story? Would two people from different countries in a fantasy world have different ideas of how the body worked and what kind of treatment would be good for it? Might one believe that washing with soap was dangerous (as we used to), while the other believed it was necessary for sanitary treatment?
Actually, it occurs to me that a wonderful example of an elaborated health concept is in Janice Hardy's book The Shifter. Given that it centers on a form of magical healing, that shouldn't perhaps come as a surprise - but it makes for a very interesting example. The healers have precise terms for talking about injuries - "breaks," "bleeds," etc. and a very extensive sense of the body and its parts. But when they heal they literally pull the pain out of the person and into themselves, and then have to get rid of it into a type of magical metal that is a finite resource. You can imagine this changes everything about the healing process... and what Janice does with it is create an entire society with a pain economy alongside (and linked to) its monetary economy. She's taking the concept to a fully elaborated extreme that influences everything about her story - worldbuilding, characters, plot, etc. And while I'm not suggesting that every author do the same, I think it's worth taking the time to consider how the people in your story think about health and the body. The example of me and my friend should make it clear that this is true even if you aren't working with fantasy and science fiction.
It's something to think about.