Actinomycosis is usually caused by an anaerobic (disliking oxygen) bacterium called Actinomyces israelii. This is a common and normally nonpathogenic (not disease-causing) organism found in the nose and throat.

Its normal location in the nose and throat account for actinomycosis, most commonly appearing in the face and neck. Because this organism is normal for people, 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 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.