The commonest cause of lung abscess -
​Lung abscess 

The term pulmonary abscess describes a local suppurative process within the lung, characterized by necrosis of lung tissues.

Etiology 

Aspiration is the most common cause, factors that portend an increased risk of aspiration include esophageal dysmotility, seizure disorders, and neurological conditions causing bulbar dysfunction.
As an aspiration of oropharyngeal secretions is the most common cause, organisms most commonly causing lung abscess are those normally found in the oral cavity, i.e., Anaerobic bacteria (Bacteroides, Fusobacterium, peptococcus species).

Other organisms are S. aureus, Klebsiella, Nocardia and gram-negative bacteria.
Morphology

Pulmonary abscesses due to aspiration are more common on the right side because of the more vertical direction of the right main bronchus.
Most often abscess is single.
The cardinal histologic change in all abscesses is the suppurative destruction of the lung parenchyma within the central area of cavitation.
Tuberculosis
Congenital
Hematogenous
Aspirated oropharyngeal secretion