It starts as a reddened, tender area
that becomes firm and hard. Eventually, the center of the abscess
softens and becomes filled with pus. There are several different types of boils depending on the location: furuncle, carbuncle, cystic acne, hidradenitis suppurativa, or pilonidal cyst. Boils could be due to an ingrown hair, breaks in the skin infected with bacteria, foreign material that has become lodged in the skin, or plugged sweat glands that become infected.