A boil is a skin infection that starts in a hair follicle or oil gland. At first, the skin turns red in the area of the infection, and a tender lump develops. After four to seven days, the lump starts turning white as pus collects under the skin.
The most common places for boils to appear are on the face, neck, armpits, shoulders, and buttocks. When one forms on the eyelid, it is called a sty. If several boils appear in a group, this is a more serious type of infection called a carbuncle.
A boil starts as a hard, red, painful lump usually about half an inch in size. Over the next few days, the lump becomes softer, larger, and more painful. Soon a pocket of pus forms on the top of the boil.