Cystic fibrosis (CF) is caused by a defective gene which causes the body to produce abnormally thick and sticky fluid, called mucus. This mucus builds up in the breathing passages of the lungs and in the pancreas, the organ that helps to break down and absorb food. This collection of sticky mucus results in life-threatening lung infections and serious digestion problems. The disease may also affect the sweat glands and a man’s reproductive system.