Actinomycosis is usually caused by an anaerobic bacteria called Actinomyces israelii, which is a common and normally not disease-causing (nonpathogenic) organism found in the nose and throat.
Because of the bacteria's normal location in the nose and throat, actinomycosis most commonly appears in the face and neck. However, the infection can sometimes occur in the chest (pulmonary actinomycosis), abdomen, pelvis, or other areas of the body. The infection is not contagious.
Symptoms occur when the bacteria enters the facial tissues after trauma, surgery, or infection. Common triggers include dental abscess or oral surgery. The infection has also been seen in certain women who have had an intrauterine device (IUD) to prevent pregnancy.
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.