A pilonidal cyst is a fluid-filled developmental defect at the base of the spine.

The terms cyst, sinus, and abscess refer to different stages of the disease process.

  • Cyst—not infected
  • Abscess—pocket of pus
  • Sinus—opening between a cyst or other internal structure and the outside

While the cyst is not serious, it can become infected and should therefore be treated. When a pilonidal cyst gets infected, it forms an abscess, eventually draining pus through a sinus. The abscess causes pain, a foul smell, and drainage.

Pilonidal Cysts
Pilonidal cyst
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A pilonidal cyst may be congenital or acquired. If congenital, it probably began as a defect that existed when you were born. Sometime later, the defect allowed an infection to develop. If acquired, it may be the enlargement of a simple hair follicle infection or the result of a hair penetrating the skin and causing an infection.