A 63 year old male with a 75 pack year history of smoking and marked chronic obstructive pulmonary disease (COPD) develops fevers and a persistent cough frequently tinged with blood. Chest x-ray reveals a complex cavitary lesion in the right upper lobe. Bronchoscopic examination identifies a large lung abscess from which pure Fusobacterium nucleatum is cultured. What is the most likely source of the Fusobacterium?