Health Questions and Answers

Question: Among immunocompetent adults, what are the major etiologic agents for community-acquired pneumonia (CAP)?


No etiologic agent is identified in over half the cases of CAP. When an etiology is found the major organisms include:

  • S. pneumoniae
  • Mycoplasma pneumoniae
  • Chlamydia pneumoniae
  • Anaerobic bacteria
  • H. influenzae
  • Legionella pneumophila
  • Respiratory viruses
  • Staphylococcus aureus

Reference: Gorbach SL, Bartlett JG, Blacklow NR (eds): Infectious Diseases, 3rd ed. Philadelphia, Lippincott Williams & Wilkins, 2004.

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