Complex sentence: Many species of the genus "Actinomyces", including "A. naeslundii", cause a chronic disease called actinomycosis, which is characterized by swelling and formation of an abscess which may exude pus.
Split into simpler sentences: Many species of the genus "Actinomyces", including "A. naeslundii", cause a chronic disease called actinomycosis. Actinomycosis is characterized by swelling and formation of an abscess which may exude pus.