Healthy lungs produce a natural antibiotic that protects them from infection by routinely killing harmful bacteria on airway surfaces  People with cystic fibroses, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic
Since the fluid on airway surfaces in the lungs of people with cystic fibrosis has an abnormally high salt concentration, scientists hypothesize that in high salt environments the antibiotic becomes ineffective at killing harmful bacteria
The salt concentration of the airway-surface fluid in the lungs of people who suffer from cystic fibrosis tends to return to its former high levels after having been reduced to levels typical of healthy lungs.