In normal lungs mucus has a vital role in maintaining normal function. Mucus traps any invading bacteria and with the aid of cilia (fine hair like structures in the lung) move the invading organisms along.
Sufferers of Cystic Fibrosis have a thicker and stickier than normal mucus. Instead of moving bacteria along it traps them in situ and causes an increase in lung infection.
Additionally; Sticky mucus blocks some of the passages in the lungs limiting the area in which gas exchange can take place.
Links
Cystic Fibrosis Trust
BBC Health
Bupa
Wellcome trust
Extension
New Scientist: Drug takes no nonsense from cystic fibrosis