Mucus is essential in the lungs, where it traps germs and pollutants that we inhale. Tiny hairs on the outside of cells, called cilia, propel the mucus out of the lungs and into the throat where the mucus can be swallowed or coughed out.
In people with cystic fibrosis, mucus is dehydrated, becoming so thick and sticky that the cilia are unable to propel mucus out of the lungs. As a result, the mucus clogs the airways, making it difficult to breathe. Because the mucus in people with CF is also abnormal in other ways, it is less able to kill germs than the mucus in healthy people, creating a fertile breeding ground for infections.