A boil is a localized infection in the skin that begins as a reddened, tender area. Over time, the area becomes firm, hard, and more tender. Eventually, the center of the boil softens and becomes filled with infection-fighting white blood cells from the bloodstream to eradicate the infection. This collection of white blood cells, bacteria, and proteins is known as pus. Finally, the pus "forms a head," which can be surgically opened or spontaneously drain out through the surface of the skin. Pus enclosed within tissue is referred to as an abscess. A boil is also referred to as a skin abscess.
There are several different types of boils:
- Furuncle or carbuncle: This is an abscess in the skin usually caused by the bacterium Staphylococcus aureus. A furuncle can have one or more openings onto the skin and may be associated with a fever or chills. The term furuncle is used to refer to a typical boil that occurs within a hair follicle. The term carbuncle is typically used to represent a larger abscess that involves a group of hair follicles and involves a larger area than a furuncle. A carbuncle can form a hardened lump that can be felt in the skin. The condition of having chronic, recurring boils is referred to as furunculosis or carbunculosis.
- Cystic acne: This is a type of abscess that is formed when oil ducts become clogged and infected. Cystic acne affects deeper skin tissue than the more superficial inflammation from common acne. Cystic acne is most common on the face and typically occurs in the teenage years.
- Hidradenitis suppurativa: This is a condition in which there are multiple abscesses that form under the armpits and often in the groin area. These areas are a result of local inflammation of the sweat glands. This form of skin infection is difficult to treat with antibiotics alone and typically requires a surgical procedure to remove the involved sweat glands in order to stop the skin inflammation.
- Pilonidal cyst: This is a unique kind of abscess that occurs in the crease of the buttocks. Pilonidal cysts often begin as tiny areas of infection in the base of the area of skin from which hair grows (the hair follicle). With irritation from direct pressure, over time the inflamed area enlarges to become a firm, painful, and tender nodule that makes it difficult to sit without discomfort. These frequently form after long trips that involve prolonged sitting.