A pilonidal cyst is a cyst at the bottom of the tailbone. Often, the cyst becomes infected, causing a pilonidal abscess. These cysts can be caused by ingrown hairs or trauma. Symptoms of a pilonidal cyst or abscess include pus, fever, swelling, redness, and pain. Pilonidal cysts need medical care -- most will be drained in a doctor's office. Follow the links below to find WebMD's comprehensive coverage about how pilonidal cysts are caused, treated, and much more.