Cystic Fibrosis. Cystic Fibrosis (CF) is an inherited disease that causes thickened mucus to form in the lungs, pancreas and other organs.In the lungs, this mucus blocks the airways, causing lung damage and making it hard to breathe.CF is a life-threatening condition, but thanks to advances in treatment and care, the average life expectancy has been steadily increasing and quality of life has improved.ystic Fibrosis. Cystic Fibrosis (CF) is an inherited disease that causes thickened mucus to form in the lungs, pancreas and other organs.