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 dangerous 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.

Cysts can occur anywhere in the body; however, some of the more common ones are listed here:

• Ganglion cyst
• Ovarian cyst
• Pilonidal cyst
• Sebaceous cyst