these infections cause scar tissue to form in the air passages. to my understnading, it is this scarring that causes the permanent loss of lung function.
it would make sense that one reason cf'ers differ so much is due to different areas of the lungs that happen to suffer the most scarring.
for example, more lung function may be lost if scarring occurs in major airways rather than the "corners" of the lungs.
and, like was mentioned already, most cf pathogens are common to the environment. you can do things to minimize exposure, but you can't eliminate it in any practical way.
certainly air filters help, as does avoiding places and activities where there are unusually high levels of cf hungry pathogens. this require research on indiviudal bugs, but it's worth it.
one strategy that i have developed is that, if i engage in a high risk activity, i squeeze a cpt session in immediatley after, or as soon as possible, along with my regularly scheduled cpt.