Questions about health are in general just fine. Maintaining health with restricted access to medical facilities is a very important part of doing outdoor activities in remote areas. Therefore I think in general health related questions are on-topic, with the exception of some non-suited cases. Those are actually very similar to general reasons for something to be off-topic, it is just more pronounced, as abuse of medical advice is dangerous.
One example is too narrow questions: Anything asking for a diagnosis of a very specific, not common-place problem, potentially even tailored to the asker as a "patient", is obviously not a good suit and should be closed.
Also anything that is a mainly a technical medical question that does not have any application in a outdoorsy scenario isn't a good suite. Yet again, many things may not seem relevant from the view of a day-trip hiker, but for someone doing a month long trek in very remote areas, it is perfectly valid. Of course there may be a limit for something to be too technical (i.e. go talk to a physician), but even then I think closing is rarely the way to go. In such situation, we should put stringent requirements on the answers by requiring factual-based arguments (i.e. citations). I.e. vote the answer saying "My grandfather several times used this her herb to stop a major internal bleed and it totally worked" into oblivion. I don't see any reason to close a question asking about whether there is anything to do about it though (obviously this is a poorly fleshed out example, a real one would have to be better).
The focus on immunity in the title is unfortunate, however the question gets apparent in the text: How to deal with stomach flu in the context of trekking in Nepal. The immunity part seems to point to a misconception about how developing a resistance to pathogenic agents works, which can absolutely be addressed by an informed layman (again, definitely including references). Still the main part that is actually practically useful to the OP and many other people is to answer this part:
How do other people deal with this kind of problem?
Which after reading the question body is a very clear question.