"Many different diseases can lead to skin conditions of very similar appearance; generally, the terms used in the table are not exclusive to a particular type of infection or disease.\n\nTerm: abscess. Definition: localized collection of pus, often with an inflamed fibrous outer capsule.\n\nTerm: bulla (pl., bullae). Definition: fluid-filled blister no more than 5 mm in diameter.\n\nTerm: carbuncle. Definition: deep, pus-filled abscess generally formed from multiple furuncles.\n\nTerm: crust. Definition: dried fluids from a lesion on the surface of the skin.\n\nTerm: cyst. Definition: encapsulated sac filled with fluid, semi-solid matter, or gas, typically located just below the upper layers of skin.\n\nTerm: folliculitis. Definition: a localized rash due to inflammation of hair follicles.\n\nTerm: furuncle (boil). Definition: pus-filled abscess due to infection of a hair follicle.\n\nTerm: macules. Definition: smooth spots of discoloration on the skin.\n\nTerm: papules. Definition: small raised bumps on the skin.\n\nTerm: pseudocyst. Definition: lesion that resembles a cyst but is not lined by a defined epithelial capsule.\n\nTerm: purulent. Definition: pus-producing; suppurative.\n\nTerm: pustules. Definition: fluid- or pus-filled bumps on the skin.\n\nTerm: pyoderma. Definition: any suppurative (pus-producing) infection of the skin.\n\nTerm: suppurative. Definition: producing pus; purulent.\n\nTerm: ulcer. Definition: break in the skin with complete loss of the epidermal barrier; open sore.\n\nTerm: vesicle. Definition: small, fluid-filled lesion.\n\nTerm: wheal. Definition: swollen, inflamed skin that itches or burns, such as from an insect bite.\n\nFigure 3.3: (a) Acne is a bacterial infection of the skin that manifests as a rash of inflamed hair follicles (folliculitis). The large whitehead near the center of the cheek is an infected hair follicle that has become purulent (or suppurative), leading to the formation of a furuncle. (b) An abscess is a pus-filled lesion. Figure description available at the end of the chapter.\n\nFigure 3.4: Numerous causes can lead to skin lesions of various types, some of which are very similar in appearance. Figure description available at the end of the chapter."