what causes cystic fibrosis]
1 Cystic fibrosis primarily affects the lungs and digestive system. 2  It leads to the production of mucus that is much thicker and more sticky than usual. 3  Cystic fibrosis is a hereditary condition, caused by a defective gene. 4  The cystic fibrosis gene must be present in both parents for a child to have the condition.