Last updated on: November 5th, 2021

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Pneumonia, community acquired



Community-acquired pneumonia (CAP) is an infection of the pulmonary parenchyma, in an immune-competent individual, that has been acquired outside of hospital. CAP is common, with an inpatient mortality rate as high as 14%.


Streptococcus pneumoniae is the organism encountered most often. Other common organisms include Haemophilus influenzae, Pseudomonas aeruginosa and Moraxella catarrhalis. Uncommon pathogens include Enterobacteriaceae, Staphylococcus aureus, and fungi such as Pneumocystis jirovecii, aspergillus or coccidioides.

Microbiology: atypical CAP

Organisms that give rise to atypical CAP include Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila and respiratory viruses.

Healthcare associated pneumonia

The 2007 guidelines of the American Thoracic Society (ATS) and Infectious Diseases Society of America (IDSA) defined an entity termed “healthcare associated pneumonia”—i.e., non-hospital acquired pneumonia in patients who had recent contact with the healthcare system. This entity does not exist in the 2019 ATS/IDSA guidelines.

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