It is an inherited disease of secretary glands, including the glands that make mucus and sweat. Cystic fibrosis mostly affects the lungs, pancreas, liver, intestines, sinuses and sex organs. Mucus is a substance made by the lining of some body tissues. In cystic fibrosis the mucous becomes thick and sticky. The mucus builds up in your lungs and blocks your airways—the tubes that carry air in and out of your lungs. The buildup of mucus makes it easy for bacteria to grow. This leads to repeated, serious lung infections. Over time, these infections can severely damage lungs.