Myositis Unusual infectious myositis An increased susceptibility to myositis of infectious aetiology, as manifested by a recurrent or severe inflammation of skeletal muscle caused by infection, or by an infectious myositis caused by an atypical or opportunistic organism. Common causative organisms include Staphylococcus aureus (pyomyositis), Streptococcus pyogenes, and Clostridium species (gas gangrene), as well as viral, fungal, and parasitic agents. Unusual infectious myositis Unusual musculoskeletal infection