Although many bacteria, viruses, fungi, and protozoa can cause CAP, most cases are caused by relatively few pathogens. In >50% of cases, a specific etiology is never determined.
- Typical bacterial pathogens include S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa.
- Atypical organisms include Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp., and respiratory viruses (e.g., influenza viruses, adenoviruses, human metapneumovirus, respiratory syncytial viruses).
- - A virus may be responsible for a large proportion of CAP cases that require hospital admission, even in adults.
- - 10-15% of CAP cases are polymicrobial and involve a combination of typical and atypical organisms.
- Involvement of anaerobes, which play a significant role in CAP only when aspiration precedes presentation by days or weeks, often results in significant empyemas.