Pilonidal disease is characterised by very small pits in the skin, typically just above the buttocks, in the midline. Although they may be entirely asymptomatic and never cause any trouble, occasionally they may trap hair (pilonidal literally means nest of hair). When this happens, the pit may become painful, swollen, red and even lead to an infection, which may require antibiotics or even a small operation to drain the abscess.
Many people I see suffer repeated attacks or develop a low-grade, "grumbling" infection or wound that continues to discharge. If it reaches this stage, a definitive operation is indicated. This involves a day-stay operation where the area of skin containing the pits is excised and the wound closed again. It sounds simple, but this part of the body is notoriously slow and difficult to heal and the process may be rather prolonged.