Aspiration pneumonia is caused by accidentally inhaling food, water, stomach acid, vomit, or another material into the lungs. The worst pneumonia results from the aspiration of stomach acid. Aspirated substances may contain bacteria, which requires treatment with an antibiotic.

Causes of aspiration pneumonia include:

• Altered level of consciousness:
• Stroke
• Alcoholism
• Drug abuse
• Seizure
• Head injury
• Disorders of the esophagus:
• Reflux esophagitis
• Esophageal stricture
• Esophageal cancer
• Neurologic illness:
• Multiple sclerosis
• Dementia
• Parkinson's disease