A cyst is an abnormal pocket of fluid that can form in many different areas of the body, including the skin, genitals, and internal organs. They are closed capsule or sac-like structures that can contain liquid, semisolid, or gaseous material, much like a blister. Cysts can be caused by injury, infection, or other issues. There are many types of cysts, including branchial cleft cysts, perineural (Tarlov) cysts, and vocal fold cysts. In most cases, cysts are not dangerous, but they may cause skin irritation, skin infection, or, in very rare adult cases, cancer. If a cyst is cancerous, treatment will depend on its size and location and on whether or not cancer cells have spread to other parts of the body. The cyst will be surgically removed along with surrounding tissue, and further treatment might include radiotherapy or chemotherapy.