A. israelii occurs as part of the normal anaerobic flora of the throat, tonsillar crypts, and mouth (particularly around carious teeth). Actinomyces may also be found in the membranes lining the intestines and vagina. Infection results from introduction of the bacteria into body tissue by a break in the integrity of the mucous membranes or weakened tissue of deeper body structures.

In cervicofacial actinomycosis (“lumpy jaw”), dental extraction or trauma to the mouth, poor dental hygiene, or periodontal disease may cause infection. Neoplasms or osteonecrosis of the jaw from radiation treatments may also be a cause. The infection may spread to the brain or bloodstream if left untreated. Cervicofacial actinomycosis is the most common type of Actinomyces infection, comprising 50% to 70% of reported cases. (See Cervicofacial actinomycosis.)

Pulmonary actinomycosis is typically caused by aspiration of bacteria from the mouth into areas of the lungs that are already anaerobic from infection or atelectasis.

In GI actinomycosis, intestinal mucosa disruption (usually by bowel surgery), ingestion of foreign bodies such as chicken bones, and inflammatory bowel conditions such as appendicitis are the most common causes of infection. Sometimes, esophageal perforation can result in infection with Actinomyces. Eventually, empyema follows, a sinus forms through the chest wall, and septicemia may occur.

The complications of actinomycosis vary, depending on the site of infection. Cervicofacial actinomycosis may progress to involve sinus and maxillofacial subcutaneous tissue. Abscesses and fistulas of the brain may also result. When the respiratory system is affected, the patient may develop pneumonia and empyema. Septicemia may occur if the infection is left untreated. Endocarditis is another reported complication, as is osteomyelitis of the jaw, ribs, and vertebrae.