Pilonidal sinus disease is a condition where small abscesses or cysts containing debris, dirt and hair form at the top of the buttocks in the cleft, as stated by Healthline. The growths cause severe pain, ooze blood and pus, and are vulnerable to infection.
Lesions that become infected swell, redden and cause pain when standing or sitting, according to Healthline. The cause of pilonidal sinus disease is unknown as of 2015, but researchers believe that changing hormones, friction from clothing and hair growth contribute to the development of cysts or abscesses. Treatment options include lancing, phenol injections into the cysts or surgery.