Interesting idea; it strikes me there's a kinda mutualism.
If outsiders are often hostile, as interlopers won't have immunities the local group have, the incomers will potentially suffer a degree of debilitation - that is perhaps sufficient to give the local group the edge in battle, or to cement friendship through nurturing the non-fatally sick.
I have a theory, more of a headcannon really, that coughing and sneezing actually evolved (in part) to deliberately spread infectious agents, as opposed to less-atomizing means of clearing mucus. Airborn viruses are typically non-lethal (otherwise it would wipe out the species readily), and it would have the effect of spreading these and creating survival pressure which culls the weak, freeing up resources and enhancing selection.
If outsiders are often hostile, as interlopers won't have immunities the local group have, the incomers will potentially suffer a degree of debilitation - that is perhaps sufficient to give the local group the edge in battle, or to cement friendship through nurturing the non-fatally sick.