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 immunocompromised hosts
      • 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.