<unk>  is often caused by a defective swallowing mechanism, such as a neurological disease or as the result of an injury that directly impairs swallowing or interferes with consciousness. Impaired consciousness can be intentional, such as the use of general anesthesia for surgery. For many types of surgical operations, people preparing for surgery are therefore instructed to take nothing by mouth (nil per os, abbreviated as NPO) for at least four hours before surgery. These conditions enable the entry of bacteria into the lungs, thus allowing the development of an infection. Risk factors * Impaired swallowing: Conditions that cause dysphagia worsen the ability of people to swallow, causing an increased risk of entry of particles from the stomach or mouth into the airways. While swallowing dysfunction is associated with<unk> , dysphagia may not be sufficient unless other risk factors are present. Neurologic conditions that can directly impact the nerves involved in the swallow mechanism include stroke, neurodegenerative diseases (such as Parkinson's disease), and multiple sclerosis. Anatomical changes in the chest can also disrupt the swallow mechanism. For example, patients with advanced COPD tend to develop enlarged lungs, resulting in compression of the esophagus and thus regurgitation. * Altered mental status: Changes in levels of consciousness affect the swallow mechanism by both disabling the body's natural protective measures against<unk>  as well as possibly causing nausea and vomiting. Altered mental status can be caused by medical conditions such as seizures. However, many other agents can be responsible as well, including general anesthesia and alcohol. * Bacterial colonization: Poor oral hygiene can result in colonization of the mouth with excessive amounts of bacteria, which is linked to increased incidence of<unk> . * Ethnicity: Asians diagnosed with<unk>  have a lower risk of death compared to other ethnic groups while African Americans and whites share a relatively similar risk of death. Hispanics have a lower risk of death than non-Hispanics. * Others: Age, male gender, diabetes mellitus, malnutrition, use of antipsychotic drugs, proton pump inhibitors, and angiotensin-converting enzyme inhibitors. Residence in an institutional setting, prolonged hospitalization or surgical procedures, gastric tube feeding, mechanical airway interventions, immunocompromised, history of smoking, antibiotic therapy, advanced age, reduced pulmonary clearance, diminished cough reflex, disrupted normal mucosal barrier, impaired mucociliary clearance, alter cellular and humoral immunity, obstruction of the airways, and damaged lung tissue. Bacteria Bacteria involved in<unk>  may be either aerobic or anaerobic. Common aerobic bacteria involved include: *Streptococcus pneumoniae *Staphylococcus aureus *Haemophilus influenzae *Pseudomonas aeruginosa * Klebsiella: often seen in<unk>  lobar pneumonia in alcoholics Anaerobic bacteria also play a key role in the pathogenesis of<unk> . They make up the majority of normal oral flora and the presence of putrid fluid in the lungs is highly suggestive of<unk>  secondary to an anaerobic organism. While it is difficult to confirm the presence of anaerobes through cultures, the treatment of<unk>  typically includes anaerobic coverage regardless. Potential anaerobic bacteria are as follows: *Bacteroides *Prevotella *Fusobacterium *Peptostreptococcus
Aspiration pneumonia
Chemical pneumonitis
Bacterial pneumonia
Tuberculosis
Viral pneumonia
Pulmonary aspiration
Occult pneumonia
Pleural empyema