A boil, also called a furuncle, is a deep folliculitis

Folliculitis

Folliculitis is the inflammation of one or more hair follicles. The condition may occur anywhere on the skin with the exception of the palms of the hands and soles of the feet.- Causes :...

, infection

Infection

An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 of the hair follicle

Hair follicle

A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....

. It is always caused by infection by the bacterium Staphylococcus aureus

Staphylococcus aureus

Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...

, resulting in a painful swollen area on the skin

Human skin

The human skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system. The skin has multiple layers of ectodermal tissue and guards the underlying muscles, bones, ligaments and internal organs. Human skin is similar to that of most other mammals,...

 caused by an accumulation of pus

Pus

Pus is a viscous exudate, typically whitish-yellow, yellow, or yellow-brown, formed at the site of inflammatory during infection. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or...

 and dead tissue. Individual boils clustered together are called carbuncle

Carbuncle

A carbuncle is an abscess larger than a boil, usually with one or more openings draining pus onto the skin. It is usually caused by bacterial infection, most commonly Staphylococcus aureus. The infection is contagious and may spread to other areas of the body or other people...

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Staphylococcus is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of bacteria that is characterized by being round (coccus or spheroid shaped), Gram-positive

Gram-positive

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

, and found as either single cells, in pairs, or more frequently, in clusters that resemble a bunch of grape

Grape

A grape is a non-climacteric fruit, specifically a berry, that grows on the perennial and deciduous woody vines of the genus Vitis. Grapes can be eaten raw or they can be used for making jam, juice, jelly, vinegar, wine, grape seed extracts, raisins, molasses and grape seed oil. Grapes are also...

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