Healthy lungs make mucus, which protects the airways and makes it easier to breathe. To make normal mucus, which is thin and watery, the body needs a special protein. This protein is defective in cystic fibrosis. So the body makes thick, sticky mucus instead, which can clog the lungs. This creates a place where bacteria can easily grow — and bacteria cause infections.

And it’s not only the airways and lungs that are affected by cystic fibrosis. Mucus-producing cells line the digestive system, including the stomach, intestines, liver, pancreas, and reproductive organs. The pancreas makes enzymes that help digest food and hormones that help absorb sugar. Thick mucus in the pancreas can make it hard for people to digest food and get all the vitamins and nutrients they need.