PATHOGENS

• S. pneumoniae and other streptococci (~50%)
  • Streptococcus intermedius complex ([previously S. milleri], S. anginosus, S. constellatus) now dominates in community-acquired cases.
• S. aureus including MRSA (10%)
• Anaerobes (20%)
• Gram-negative bacilli (GNB, 10%)
• Noscomial: Gram-negative bacilli and S. aureus (MRSA)
  
• Rare causes:
  • Mycobacterium tuberculosis
  • Enterococcus
  • Actinomyces
  • Other:
    • Viruses are a highly unusual cause except for influenza complicated by bacterial superinfection.
    • Fungi: rare except in immunocompromised hosts or those with malignancy.
      • Candida spp.
      • Aspergillus
      • Cryptococcus
    • Parasites:
      • Echinococcus
      • Entamoeba histolytica
      • Paragonimus westermani
• Major causes in CAP: S. pneumoniae, streptococcal species, S. aureus and anaerobic bacteria (polymicrobial).
  • Fulminant course in healthy host: usually MRSA, especially if following influenza as a bacterial superinfection.