A boil (furuncle) is a red, swollen, painful bump under the skin caused by an infected hair follicle. Bacteria from the infection forms a pocket of pus (abscess), which can become large and cause severe pain.
Boils occur most often on the face, neck, breasts, buttocks, in the groin area, and in the armpits. Depending on their size and location, many boils can be treated at home with warm compresses and by keeping the skin over the boil clean when it opens and drains. A doctor may need to open large boils and boils on the face or near the spine or anal area to avoid more serious infections.