Why does cystic fibrosis increase the risk of lung infections?

Cystic fibrosis is a genetic disorder of the CFTR protein, which alters the salt and water balance in the body fluids. Specifically in the lungs, this causes the mucous in the airways to retain less water, making it thicker and much more sticky (viscous). The airways have specialised hair-like structures called cilia, which normally act to move mucous containing bacteria out of the airways. When the mucous is more viscous, the cilia are unable to move this mucous out of the airways, and so it (and all the bacteria it retains) is trapped in the airway.