Cystic fibrosis is a genetic disease that causes thick, sticky mucus to build up in the lungs and digestive system. It is caused by a defective gene that results in abnormally thick mucus. Symptoms often seen in childhood include delayed growth, failure to gain weight, salty-tasting skin, and weight loss. While most cases were diagnosed by age 10 in the 1950s, treatments developed by the Cystic Fibrosis Foundation have helped people with cystic fibrosis now live into their 30s and 40s.