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Etiology

• Most cases of CAP are caused by a few common respiratory pathogens, including:
  • Streptococcus pneumoniae
    • Accounts for ~50% of all cases of CAP requiring hospital admission
  • Haemophilus influenzae
  • Staphylococcus aureus
  • Mycoplasma pneumoniae
  • Chlamydia pneumoniae
  • Moraxella catarrhalis
  • Legionella spp.
  • Aerobic gram-negative bacteria
  • Influenza viruses
  • Adenoviruses
  • Respiratory syncytial virus
  • Other rare organisms
    • Viral: hantavirus, Nipah virus, Hendra virus, metapneumovirus, severe acute respiratory syndrome (SARS) virus
    • Nonviral: Pneumocystis, Mycobacterium tuberculosis , fungi, bioterrorism agents (e.g., those of Q fever, tularemia, anthrax, plague), etc.
• The relative frequency of these pathogens differs with the age of the patient and the severity of the pneumonia.
• Pathogenesis
  • Microaspiration of oropharyngeal secretions colonized with pathogenic microorganisms (e.g., S. pneumoniae, H. influenzae) is the most common route.
  • Gross aspiration
    • Central nervous system disorders that affect swallowing (e.g., stroke, seizures)
    • Impaired consciousness (e.g., in alcoholism, IV drug use)
    • Anesthesia or intubation
    • Pathogens include anaerobic organisms and gram-negative bacilli.
  • Aerosolization (e.g., of M. tuberculosis, Legionella spp., viruses)
  • Hematogenous spread (e.g., seeding of the lungs by S. aureus during endocarditis)
  • Contiguous spread from another site

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