INTRODUCTION

Community-acquired pneumonia (CAP) is defined as an infection of the pulmonary parenchyma, acquired in the community. The definition of CAP excludes patients who are hospitalized, have been hospitalized in the 14 days prior to the onset of symptoms, or who reside in long-term care facilities, including nursing homes. This chapter focuses on CAP in immunocompetent adults. See Chapter 35, HIV-Associated Respiratory Infections, for a discussion of pulmonary infections in immunocompromised patients and Chapter 49, Pediatric Respiratory Infections, for a discussion on pediatric pulmonary infections.

Streptococcus pneumoniae is the most important CAP pathogen, accounting for 35–55% of cases of CAP, and two-thirds of deaths from CAP. Current S. pneumoniae resistance to both to penicillins and macrolides, as well as the possibility of future widespread resistance to fluoroquinolones, drives current recommendations for empiric therapy. The risk factors for infection with drug-resistant S. pneumoniae, include significant medical comorbidities and use of antimicrobials within the prior 3 months.

The so-called atypical bacterial causes of CAP, which cannot be seen on Gram stain or cultured on typical media, include Mycoplasma pneumoniae, Chlamydophila (formerly Chlamydia) pneumoniae, and Legionella pneumophila. M. pneumoniae and C. pneumoniae are common causes of CAP in ambulatory patients younger than 50. No rapid diagnostic tests exist for M. pneumoniae and C. pneumoniae, and although they are associated in general with a less severe disease, pneumonia caused by these organisms cannot be distinguished on clinical grounds.