A cyst is a small, slow-growing, noncancerous bump beneath the skin. It is a sac-like structure, usually filled with liquid, within the body’s tissue. Cysts are fairly common and can appear anywhere under the skin. Cysts usually form due to infections, inherent diseases, chronic inflammation or blockages in ducts. Most cysts need medical attention for them to go away. Cysts can be drained or injected with cortisone medication, causing them to shrink but many cysts may need to be surgically removed to avoid reappearance. Whether a cyst needs treatment depends on the type of cyst, location of the cyst, potential pain, and whether the cyst is infected.