A pilonidal cyst is a growth on the bottom of the coccyx, or tailbone, according to WebMD. If the cyst develops an infection, it is known as a pilonidal abscess and can fill with pus. When it becomes an abscess, it resembles a pimple just over the crack between buttocks.
Pilonidal cysts generally happen to people up through their thirties and are more frequent in men than women. Most physicians believe that the cause for these cysts is ingrown hair. Indeed, the meaning of pilonidal includes "nests of hair," and follicles are frequently found within the cyst, as stated by WebMD.
Other physicians believe that trauma to the coccyx or the surrounding region causes pilonidal cysts. During World War II, over 80,000 soldiers developed these cysts and required hospitalization. The condition was called "Jeep disease" for a time because people thought that being jolted around in Jeeps was the cause for the irritation, reports WebMD.
Symptoms of a pilonidal cyst include fever, draining pus, redness, and pain and swelling at the base of the spine. If it develops into an infected abscess, it requires lancing or draining to get better. Typical with boils, these do not get better as a result of taking an antibiotic regimen, according to WebMD.