A boil is a localized bacterial infection of a hair follicle. Nearly all boils are caused by Staphy-lococcus aureus, a bacterium that produces a number of locally harmful toxins.
During the stage of infectious invasion, the microorganism penetrates the epidermis of the follicle, starts to multiply, and produces a toxin that damages or kills the surrounding cells. The dead tissue then liquefies and forms pus, a mixture of dead skin cells, dead white blood cells, and both living and dead bacteria in tissue fluid. The presence of Staphylococcus aureus attracts the white blood cells to the area. However, the bacteria are somewhat protected from being engulfed by the white cells by an enzyme that they produce. In addition, the bacteria produce a leucocidin that kills white blood cells. Once the accumulation of pus is visible to the naked eye, an abscess is said to exist. Although abscesses may occur in any organ of the body, the term "boil" is reserved for an abscess in a hair follicle.
A boil, as any abscess, may act as a focus from which bacteria are disseminated through the bloodstream to other parts of the body. Ordinarily, the body's normal defense mechanisms prevent the infection from spreading, but an external disturbance may allow the microorganisms to spread. This is why the removal of an infected hair or the squeezing of a pimple (which is actually a very small boil) may spread the infection.
Some small boils, called blind boils, do not form a visible core of pus and heal spontaneously in a short period of time. In most boils, however, the epidermis overlying the pus liquefies, allowing the pus to escape. The thinning and bulging of the epidermis prior to the actual rupture is known as pointing, and this process may be hastened by the local application of heat. Once the pus escapes, the surrounding inflammation diminishes and healing begins. If a boil is slow to rupture, it may be surgically incised to permit drainage. However, it is important that a boil not be incised prematurely, since the infection may be spread in this manner. Sometimes, boils may be treated by the administration of antibiotics. Although individual boils are not difficult to treat, they often recur because the causative bacteria remain in the patient's nose and throat.