Actinomycosis is usually caused by an anaerobic (disliking oxygen) bacterium called Actinomyces israelii, which is a common and normally nonpathogenic (not disease-causing) organism found in the nose and throat. Because of the bacterium's normal location in the nose and throat, actinomycosis most commonly appears in the face and neck. And since it is normal for people to carry this organism, the infection is not contagious. Actinomyces produces disease when it is introduced into the facial tissues by trauma, surgery, or infection. A common cause is dental abscess or oral surgery. Once in the tissue, it forms an abscess, producing a hard, red-to-reddish-purple lump, often on the jaw, from which comes the condition's common name, "lumpy jaw." Eventually, the abscess breaks through the skin surface to produce a draining sinus tract. Actinomycosis may occasionally occur in the chest (pulmonary actinomycosis) and abdomen or other areas of the body.
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