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Pneumonia is an infection of the pulmonary parenchyma. Despite being the cause of significant morbidity and mortality, it is often misdiagnosed, mistreated, and underestimated. Pneumonia historically was typically classified as community-acquired (CAP), hospital-acquired (HAP), or ventilator-associated (VAP). A fourth category, health care–associated pneumonia (HCAP), was introduced recently. This category was meant to encompass those cases of CAP that were caused by multidrug-resistant (MDR) pathogens typically associated with HAP. Unfortunately, the original definitions appear to have been overly sensitive, resulting in the treatment of a high proportion of patients who had community-onset pneumonia with broad-spectrum antibiotics consistent with HAP treatment. Retrospective studies have actually suggested a worse outcome when broad-spectrum antibiotics were used in these cases.

Rather than relying on a predefined subset or category of pneumonia cases, it is likely to be of greater value to assess each case individually on the basis of risk factors for infection with an MDR organism. Rather than originating in primary pneumonia research, the original HCAP definition was modified from a study of health care–associated bacteremia. Recent studies have more closely identified patients at risk for pathogens resistant to the antibiotics usually used; have defined risk factors for infection with methicillin-resistant Staphylococcus aureus (MRSA) independent of other MDR pathogens; and have found that at least two, if not three, risk factors are required before the probability of drug-resistant pathogens is sufficient to influence initial empirical broad-spectrum antibiotic therapy. These risk factors are listed in Table 121-1.

TABLE 121-1Risk Factors for Pathogens Resistant to Usual Therapy for Community-Acquired Pneumoniaa

This chapter deals with pneumonia in patients who are not considered to be immunocompromised. Pneumonia in severely immunocompromised patients, some of whom overlap with the groups of patients considered in this chapter, warrants separate discussion (see Chaps. 70, 138, and 197).


Pneumonia results from the proliferation of microbial pathogens at the alveolar level and the host’s response to those pathogens. Microorganisms gain access to the lower respiratory tract in several ways. The most common is by aspiration from the oropharynx. Small-volume aspiration occurs frequently ...

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