A cyst is a closed sac having a distinct membrane and developing abnormally in a cavity or structure of the body. Cysts can often be dangeorus as they may have negative effects on the nearby tissue. They may contain air, fluids, or other materials.
Cystic fibrosis is an example of a genetic disorder whereby cysts develop in lung tissue and release mucus into the lungs (see Alveoli) reducing lung capacity and causing persistent coughing.