A furuncle usually starts as a small, firm, red lump under the skin which will be tender to touch. The most common areas for boils to occur are places where there is friction and/or places that tend to be sweaty, such as the buttocks, thighs, armpits, groin, neck, shoulders, and face. The skin surrounding the lump may look swollen and red. It may become painful, swollen, and filled with yellow or white pus. A boil in the ear canal may be very painful.
The infection is usually caused by a type of bacteria called Staphylococcus aureus ("staph"). Many people are "carriers" of the staph germ it normally lives on their skin or in their nose without doing them any harm. Tiny breaks in the surface of the skin (such as those caused by friction or scratching), however, can help the germ gain entry into and infect the hair follicle, resulting in a boil.