Commonest cause of lung abscess -
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.
Aspiration
Hematogenous spread from distant site
Direct contact
Lymphatic spread