A furuncle (boil) is a skin infection involving an entire hair follicle and the adjacent subcutaneous tissue.

Furuncles are very common. They are caused by staphylococcus bacteria, which are normally found on the skin surface. Damage to the hair follicle allows these bacteria to enter deeper into the tissues of the follicle and the subcutaneous tissue. Furuncles may occur in the hair follicles anywhere on the body, but they are most common on the face, neck, armpit, buttocks, and thighs.

Furuncles are generally caused by Staphylococcus aureus, but they may be caused by other bacteria or fungi. They may begin as a tender red subcutaneous nodule but ultimately become fluctuant (feel like a water-filled balloon). A furuncle may drain spontaneously, producing pus. More often the patient or someone else opens the furuncle.

Furuncles can be single or multiple. Some people have recurrent bouts with abscesses and little success at preventing them. Furuncles can be very painful if they occur in areas like the ear canal or nose. A health care provider should treat furuncles of the nose. Furuncles that develop close together may expand and join, causing a condition called carbunculosis.