(See also Neonatal Pneumonia Neonatal Pneumonia Neonatal pneumonia is lung infection in a neonate. Onset may be within hours of birth and part of a generalized sepsis syndrome or after 7 days and confined to the lungs. Signs may be limited... read more .)
An estimated 4 to 5 million people in the United States develop pneumonia (excluding COVID-19 pneumonia) each year, of whom about 55,000 die. In the United States, pneumonia, along with influenza Influenza Influenza is a viral respiratory infection causing fever, coryza, cough, headache, and malaise. Mortality is possible during seasonal epidemics, particularly among high-risk patients (eg, those... read more , is the 9th 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 medically underserved countries.
The most common cause of pneumonia in adults > 30 years, in the absence of an ongoing viral pandemic such as COVID-19, is
Streptococcus pneumoniae is still the most common pathogen in all age groups, settings, and geographic regions. However, the incidence of S. pneumoniae infection has been declining because of increasing rates of vaccination and development of herd immunity. However, pathogens of every sort, from viruses to parasites, can cause pneumonia. With the availability of sensitive molecular diagnostic methods, there is increasing recognition of viruses as a cause of pneumonia, either as a sole pathogen or as a co-pathogen with bacteria.
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. Another potential pathway of pneumonia development is dysbiosis in the resident microbiome of the lung.
Upper airway defenses include salivary IgA, proteases, and lysozyme; 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, disorders or drugs that suppress the immune system). Pathogens that then reach airspaces can multiply and cause pneumonia.
Specific pathogens causing pneumonia cannot be found in up to 50% of patients, even with extensive diagnostic investigation, primarily because of the limitations of currently available diagnostic tests and the difficulty of obtaining adequate deep lung (alveolar) samples. But because pathogens and outcomes tend to be similar in patients in similar settings and with similar risk factors, it is clinically useful to categorize pneumonias as
Occurring in immunocompromised patients Pneumonia in Immunocompromised Patients Pneumonia in immunocompromised patients is often caused by unusual pathogens but may also be caused by the same pathogens that cause community-acquired pneumonia. Symptoms and signs depend on... read more , including patients with human immunodeficiency virus (HIV) infection (see Pneumocystis jirovecii Pneumonia Pneumocystis jirovecii Pneumonia Pneumocystis jirovecii is a common cause of pneumonia in immunosuppressed patients, especially in those infected with human immunodeficiency virus (HIV) and in those receiving systemic... read more )
Aspiration pneumonia Aspiration Pneumonitis and Pneumonia Aspiration pneumonitis and pneumonia are caused by inhaling toxic and/or irritant substances, most commonly large volumes of upper airway secretions or gastric contents, into the lungs. Chemical... read more , which occurs when large volumes of upper airway or gastric secretions enter into the lungs
These categorizations allow treatment to be selected empirically.