In CF, then, you have an interesting situation. Because of the GSH efflux problem in CF, you have (among other things) a compromised immune system that is not as effective in combating pathogens. So you get things like fungus balls and cepacia. Dr. Visca has found that CF patients who take GSH start to culture less quantity of bacteria, and even, over time, culture fewer species of bacteria. So it is clear that taking GSH helps CF patients combat pathogens.
On the other hand, it is also true that the accepted medical treatment for some of these pathogens is to use things like antibiotics and fungicides to kill them. Bacteria and fungus can use any extra GSH in their environment to help protect themselves from these treatments.
So, again from Dr. Visca's results, on balance it seems that GSH helps CF persons fight pathogens and should be used. However, the calculus becomes a little more complex when we are talking about a potentially life-threatening pathogen. Both cepacia and fungus balls are dangerous enough that one might ask whether it is not better to try and vigorously eradicate the pathogen before using GSH, or at least use oral GSH and not inhaled GSH. This is something that must be discussed with your doctor.
Cepacia can, under the right circumstances, go quickly septic, causing "cepacia syndrome," which can lead to death within a few days. Fungus balls can sometimes grow large enough to obstruct airway flow in the lung and be very dangerous. So a lot of pondering and discussing and talking with doctors should occur for patients that have dangerous conditions in CF.