Commonest cause of lung abcess -

Routes of infection to lung Aspiration of organisms that colonize the oropharynx (MC) Inhalation of infections, aerosols Hematogenous dissemination from an extrapulmonary site. Direct inoculation (as in tracheal intubation or stab wounds) Contiguous spread from an adjacent site of infection. Ref : Harrison 17/e p1631