The lungs make mucus to help defend against germs. Cystic fibrosis changes the mucus, making it thick and hard to clear. This mucus is where infections can occur. Infections cause inflammation or swelling of the lungs. Both infections and inflammation cause more mucus to be made. More mucus in the lungs can lead to more infections. This cycle of infection, inflammation and more mucus can hurt the lungs and lower lung function.