A Pilonidal Sinus is an infected tract under the skin between the buttocks. Treatment commonly involves an operation. After the operation you should keep the area free of growing hair by regular shaving or other means of hair removal.
The exact cause of this condition isn’t known, but its cause is believed to be a combination of changing hormones (because it occurs after puberty), hair growth, and friction from clothes or from spending a long time sitting.
Activities that cause friction, like sitting, can force the hair growing in the area to burrow back under the skin. The body considers this hair foreign and launches an immune response against it, similar to how it would react when dealing with a splinter. This immune response forms the cyst around your hair. Sometimes a person may have multiple sinuses that connect under the skin.