A boil is a localized infection in the skin that begins as a reddened, tender area. Over time, the area becomes firm, hard, and increasingly 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 may 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. Boils can occur anywhere on the body, including the trunk, extremities, buttocks, groin, or other areas.