An abscess is a localized collection of pus. A minute abscess is known as a pustule; a diffused production of pus is known as cellulitis or erysipelas. An abscess may be acute or chronic. Chronic abscesses are often tuberculous, caused by Mycobacterium tuberculosis. The direct cause of an acute abscess is bacteria, most frequently Staphylococcus spp and Streptococcus spp, the latter causing more virulent abscesses. Pseudomonas pyocyanea produces blue or greenish pus, and Escherichia coli, which is always present in the bowels, may under certain conditions wander into surrounding tissues and produce abscesses. Foreign bodies, such as bullets or splinter, may initiated abscesses if infectious microorganisms are also present. However, the mere presence of microorganisms will not produce abscesses; infection depends on the resistance of the individual, that is the strength of their immune system.
In 2000, Mycobacterium fortuitum contaminated a footbath at a California pedicurist's salon and caused boils and lesions on the legs of as many as 108 women and one man. Most people infected in the outbreak had multiple sores or boils, one woman had 37.