Aspiration pneumonia is a specific form of pneumonia that develop when gastric contents, saliva or nasal secretions are aspirated into the bronchial tree. Depending on the acidity of the aspirate, a chemical pneumonitis develops, and bacterial pathogens (including anaerobic bacteria) further add to the inflammation.

Aspiration is often caused by an incomplete swallowing reflex, such as occurs in some forms of neurological disease (a common cause being strokes).

The right lower lobe of the lung is the commonest location of aspiration pneumonia. This is due to the anatomy of the bronchial tree and gravity: the bronchus serving this lobe is oriented almost vertically.