I believe the term that's used for symptoms that help you narrow down the possible causes is "differential". "A tummy ache is not terribly differential. The presence of a screwdriver extending from the wound is pretty differential." I think there are degrees of how differential symptoms are.
If you have multiple symptoms, they can help to narrow down the possible cause by being considered together. Weakness isn't terribly differential, nor is nausea on its own, but if you combine the two, you move in the direction of narrowing things down. Indeed, the lack of symptoms can be used to help diagnose, too. Sensitivity to light suggests a lot of things, and lacking a malar rash helps to start to rule out lupus, for example.
I have ideas for handling illnesses that aren't well document or understood.
I think it should be possible to create a program, a website, where you can enter your symptoms and then test out different remedies and report your success or failure, and thus collectively, with many users, generate statistics that help diagnose and treat illness.
"Your symptoms match very well with this particular cluster of symptoms. Other people who matched well with this cluster of symptoms found improvement with the following treatments..."