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Cystic fibrosis is caused by a changed (mutated) gene that a child inherits from both parents. The changed gene causes problems with the way salt and water move in and out of the cells that make mucus. This causes the mucus to be thick and sticky. This glue-like mucus builds up and causes problems in many of the body's organs, especially the lungs and the pancreas.
Many people don't know they have the changed gene.