A boil, or skin abscess, is a localized infection deep in the skin. A boil generally starts as a reddened, tender area. Over time, the area becomes firm and hard. Eventually, the center of the abscess softens and becomes filled with white cells that the body sends to fight the infection. This collection of white cells is known as pus. Finally, the pus "forms a head" and drains out through the skin. Skin abscesses may occur anywhere on the body. They affect people of all ages.

There are several different types of boils. Among these are:
Furuncle or carbuncle: This is an abscess in the skin caused by the bacteria Staphylococcus Aureus. It can have one or more openings onto the skin and may be associated with a fever or chills.

Cystic acne: This is a type of abscess formed when oil ducts become clogged and infected. Cystic acne is most common in the teenage years. 

Hidradenitis suppurativa: This is an illness in which there are multiple abscesses that form under the arm pits and in the groin area. These are a result of local inflammation of the sweat glands. 

Pilonidal cyst: This is a special kind of abscess that occurs in the crease of the buttocks. These frequently form after long trips that involve sitting.