Actinomycosis is caused by anaerobic or microaerophilic bacteria primarily of the genus Actinomyces (e.g., A. israelii). This diagnosis should be considered when a chronic progressive process with mass-like features crosses tissue boundaries, a sinus tract develops, and/or the pt has evidence of a refractory or relapsing infection despite short courses of antibiotics. Most infections are polymicrobial, but the role of other species in the pathogenesis of the disease is unclear.