Cross-section of a boil. Following bacterial infection of the hair follicle, there is a buildup of pus, and a raised tender lump appears on the surface of the skin.
A boil is an abscess in a hair follicle, usually caused by infection with staphylococcus. A sty is a boil on the eyelid; a carbuncle is a group of contiguous boils. Small boils may heal spontaneously, but most cannot until the pus has escaped by thinning and rupture of overlying skin. This is hastened by local application of heat. In severe cases lancing and antibiotics may be required.