Light micrograph section through a human lung tissue showing evidence of cystic fibrosis. Cystic fibrosis is a hereditary disease that affects the lungs and digestive system. The body produces thick and sticky mucus that can clog the lungs and obstruct the pancreas. Cystic fibrosis (CF) occurs in a child when both parents have the defective gene. Although there is no known cure, good nutrition helps to thin the mucus and improve mucus expectoration.