A pilonidal sinus is a small cavity which develops under the skin in the area of the 'natal cleft' (which is in the centre of the lower back where the cleft of the buttocks begins).
Pilonidal sinuses are caused by body hair on the back, or in the natal cleft itself, growing into the skin and effectively burrowing under the surface of the skin. If this small cavity this creates becomes infected it can become swollen and painful, sometimes resulting in the discharge of pus and/or blood.
People in their teens and early twenties are much more affected by the condition than people in other age groups. The condition is also referred to as 'pilonidal disease'.
Where symptoms are severe or where infection is a regular occurrence, surgery may be required.