Major risk factor for development of lung abscess is:

The major risk factor for primary lung abscesses is aspiration. Patients at particular risk for aspirationsuch as those with altered mental status, alcoholism, drug overdose, seizures, bulbar dysfunction, prior cerebrovascular or cardiovascular events, or neuromuscular disease, are most commonly affected.  Lung abscesses are usually characterized as either primary (~80% of cases) or secondary.  Primary lung abscesses usually arise from aspiration, are often caused principally by anaerobic bacteria, and occur in the absence of an underlying pulmonary or systemic condition.  Secondary lung abscesses arise in the setting of an underlying condition, such as a post obstructive process  (e.g., a bronchial foreign body or tumour) or a systemic process (e.g., HIV infection or another immunocompromising condition).  Lung abscesses can also be characterized as acute (<4–6 weeks in duration) or chronic (~40% of cases). Reference: Harrison’s PRINCIPLES OF INTERNAL MEDICINE 20th edition page no 919