A boil is a skin infection that begins in a hair follicle or oil gland, often through a germ that enters through a small nick or cut in the skin. They most commonly appear on the face, neck, shoulders, armpits, and buttocks.
The first sign of a boil is the sudden appearance of red skin at the area of infection, followed by a tender lump. After a few days, pus will begin collecting under the skin and the lump will turn white.
Although boils do not usually require immediate emergency attention, you should seek medical care if the boil does not drain or if you develop a fever and chills. Both of these could be a sign of infection.