Aspiration pneumonia is usually caused due to a faulty swallowing mechanism that occurs in some cases of neurological disease (eg strokes) or in case of intoxication. It could also occur as an iatrogenic cause during surgery, under the influence of general anesthesia. Therefore patients are advised not to eat (NPO) atleast 4 hours prior to surgery.
It is still a matter of speculation and controversy as to whether aspiration pneumonia is a bacterial infection or a chemical inflammatory condition. Both causes show similar symptoms.
Disorders of the oesophagus, (gastroesophageal reflux, esophageal stricture), or a low or absent gag reflex in unconscious or semi conscious persons could induce the aspiration of foreign material (often stomach contents) into the lungs. Other factors contributing to this risk include old age, dental problems, use of sedatives, coma and excessive consumption of alcohol. The extent of injury to the lungs depends on the type and quantity of the inhaled substance. The greater the acidity of the substance, the greater is the extent of damage to the lung, although this may not actually lead to pneumonia.
The injured lungs could get infected with species of anaerobic and aerobic bacteria. This could lead to the collection of pus or abscess in the lung around which a protective membrane may form.