Aspiration of foreign material (often the stomach contents) into the lung can be a result of disorders that affect normal swallowing, disorders of the esophagus (esophageal stricture, gastroesophageal reflux), or decreased or absent gag reflex (in unconscious, or semi-conscious individuals). Old age, dental problems, use of sedative drugs, anesthesia, coma, and excessive alcohol consumption are also causal or contributing factors. The response of the lungs depends upon the characteristics and amount of the aspirated substance. The more acidic the material, the greater the degree of lung injury, although this may not necessary lead to pneumonia.

The injured lungs can become infected with multiple species of anaerobic bacteria or aerobic bacteria. A collection of pus may form in the lung. A protective membrane may form around the abscess.