Pilonidal disease is a chronic infection of a cyst (pilonidal cyst) that develops near the top of the crease between the buttocks. When the infected cyst-also called a pilonidal abscess-in the skin does not heal, it is called pilonidal disease.
The cause of a pilonidal cyst is not clear. It may be caused by a hair follicle in the skin that becomes blocked and then infected. Continued exercise or walking can pull hair into the infected cyst, forming an abscess. Treatment may include keeping the area clean, draining the cyst, and sometimes surgery to remove the cyst if it does not clear up on its own.
WebMD Medical Reference from Healthwise