Merck Manual

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Overview of Pneumonia


Sanjay Sethi

, MD, University at Buffalo SUNY

Last full review/revision Mar 2019| Content last modified Mar 2019
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Pneumonia is acute inflammation of the lungs caused by infection. Initial diagnosis is usually based on chest x-ray and clinical findings. Causes, symptoms, treatment, preventive measures, and prognosis differ depending on whether the infection is bacterial, mycobacterial, viral, fungal, or parasitic; whether it is acquired in the community or hospital; whether it occurs in a patient treated with mechanical ventilation; and whether it develops in a patient who is immunocompetent or immunocompromised.

(See also Neonatal Pneumonia.)

An estimated 2 to 3 million people in the United States develop pneumonia each year, of whom about 60,000 die. In the United States, pneumonia, along with influenza, is the 8th leading cause of death and is the leading infectious cause of death. Pneumonia is the most common fatal hospital-acquired infection and the most common overall cause of death in developing countries.

The most common cause of pneumonia in adults > 30 years is

  • Bacterial infection

Streptococcus pneumoniae is the most common pathogen in all age groups, settings, and geographic regions. However, pathogens of every sort, from viruses to parasites, can cause pneumonia.

The airways and lungs are constantly exposed to pathogens in the external environment; the upper airways and oropharynx in particular are colonized with so-called normal flora. Microaspiration of these pathogens from the upper respiratory tract is a regular occurrence, but these pathogens are readily dealt with by lung host defense mechanisms. Pneumonia develops when

  • Defense mechanisms are compromised

  • Macroaspiration leads to a large inoculum of bacteria that overwhelms normal host defenses

  • A particularly virulent pathogen is introduced

Occasionally, infection develops when pathogens reach the lungs via the bloodstream or by contiguous spread from the chest wall or mediastinum.

Upper airway defenses include salivary IgA, proteases, and lysozymes; growth inhibitors produced by normal flora; and fibronectin, which coats the mucosa and inhibits adherence.

Nonspecific lower airway defenses, including cough and mucociliary clearance, prevent infection in airspaces. Specific lower airway defenses include various pathogen-specific immune mechanisms, including IgA and IgG opsonization, antimicrobial peptides, anti-inflammatory effects of surfactant, phagocytosis by alveolar macrophages, and T-cell–mediated immune responses. These mechanisms protect most people against infection.

Numerous conditions alter the normal flora (eg, systemic illness, undernutrition, hospital exposure, antibiotic exposure) or impair these defenses (eg, altered mental status, cigarette smoking, nasogastric or endotracheal intubation). Pathogens that then reach airspaces can multiply and cause pneumonia.

Specific pathogens causing pneumonia cannot be found in < 50% of patients, even with extensive diagnostic investigation, primarily because of the limitations of currently available diagnostic tests. But because pathogens and outcomes tend to be similar in patients in similar settings and with similar risk factors, pneumonias can be categorized as

These categorizations allow treatment to be selected empirically.

The term interstitial pneumonia refers to various unrelated conditions of varied and sometimes unknown causes characterized by inflammation and fibrosis of the pulmonary interstitium.

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