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Pneumonia

*Pneumonia*

For medical students2 min readUpdated 2026-10-10

Pneumonia represents a group of acute inflammatory processes in the lung tissue of infectious origin, characterized by diverse clinical and morphological manifestations and pathogenesis. These conditions are leading causes of disability and pose a high health threat to neonates and elderly populations.

Mortality in the US3rd leading cause of death together with influenza

Nosocomial casesDevelop no earlier than 48 hours after hospital admission

Frequency of lobar pneumoniaAccounts for 0.9% of the incidence structure in regional studies

Risk factorsSmoking, alcohol use, immunodeficiencies, and viral respiratory infections

Primary Routes of Infection

Microorganisms penetrate lung tissue via four main routes:

Airborne (Inhalation) — inhaled directly with inspired air.
Aspiration — from the nasopharynx and oropharynx.
Hematogenous — from infection foci located in other parts of the body.
Contact — via direct extension of inflammation from adjacent affected areas.

Airborne and aspiration routes are of primary significance in the development of pulmonary inflammation, especially when the natural protective barriers of the lungs are compromised.

Pathogenetic Classification

According to the mechanism of onset, pneumonias are divided into two major groups:

Primary pneumonias present as independent diseases and develop in individuals without prior pulmonary pathology (e.g., lobar pneumonia or Legionnaires' disease).
Secondary pneumonias occur significantly more frequently. They accompany systemic diseases, chronic respiratory pathologies, or infections with a primary focus outside the lungs. This category also includes all nosocomial (hospital-acquired) variants.

Nosocomial (Hospital-Acquired) Pneumonia

Such pathologies are diagnosed later than 48 hours after a patient's admission to the hospital. Their overall share among all hospitalized patients is 8.5%; in intensive care units, this figure rises to 13%, and in oncology chemotherapy units, it reaches 30%.

The primary causative agents of hospital-acquired infections are most frequently Gram-negative aerobic bacteria, including Klebsiella, Pseudomonas aeruginosa, Escherichia coli, Proteus, as well as staphylococci and streptococci. The disease is often characterized by mixed flora.

Interstitial Inflammation and Morphological Forms

In interstitial inflammation, the pathological process involves the alveolar wall and the interstitial tissue of the respiratory zones, while secondary exudate accumulation occurs within the alveolar lumina. Such states are frequently referred to as acute pneumonitis or atypical pneumonia.

The clinical and morphological classification distinguishes three main forms:

Lobar (croupous) pneumonia
Bronchopneumonia (focal pneumonia)
Acute interstitial pneumonia (acute pneumonitis)

Mnemonic

The four routes of infection (Airborne, Aspiration, Hematogenous, Contact) can be remembered using the acronym A-A-H-C ("All Airways Have Contamination").

Frequently asked questions

What are the stages of pathogenesis in lobar pneumonia?

Lobar pneumonia exhibits four classic pathomorphological stages:

Stage of congestion (engorgement) — rapid filling of alveoli with fibrinous exudate containing pneumococci.
Stage of red hepatization — exudate is dominated by erythrocytes with a small admixture of neutrophils.
Stage of gray hepatization — massive influx of neutrophils performing phagocytosis of bacteria.
Stage of resolution — resorption of exudate and restoration of lung tissue architecture (sometimes combined with convalescence).

What pulmonary complications are typical for lobar pneumonia?

Pulmonary complications of lobar pneumonia include destructive processes and impaired organization of the exudate.

Carnification — organization of unabsorbed fibrinous exudate by connective tissue, resulting in pulmonary fibrosis of the lobe.
Lung abscess — arises from excessive neutrophil activity and secondary pyogenic superinfection.
Lung gangrene — develops upon the addition of putrefactive flora.
Empyema of the pleura — suppurative inflammation of fibrinous pleural deposits.

Other pulmonary complications include exudative pleuritis and acute respiratory distress.

What extrapulmonary complications can develop in lobar pneumonia?

Extrapulmonary complications of lobar pneumonia are associated with lymphogenous or hematogenous dissemination of the infection.

Lymphogenous — purulent mediastinitis and purulent pericarditis.
Hematogenous — metastatic brain abscesses, purulent meningitis, acute ulcerative or polypoid-ulcerative endocarditis (most commonly affecting the tricuspid valve), purulent arthritis, peritonitis, and sepsis.

Bacteremia in lobar pneumonia is documented in 30% of cases.

What microscopic changes are observed in the alveoli during the stage of gray hepatization?

During the stage of gray hepatization, microscopic examination of the alveoli reveals an accumulation of specific inflammatory exudate and microvascular changes.

Vascular changes — collapse (compression) of pulmonary capillaries is noted.
Exudate composition — alveoli are entirely filled with fibrinous exudate containing a high concentration of living and dead polymorphonuclear leukocytes (neutrophils) and macrophages.
Cellular functions — massively recruited granulocytes perform phagocytosis of opsonized pneumococci and lysis of fibrin, while macrophages phagocytize necrotic debris.

Which diseases and conditions most commonly become complicated by secondary pneumonia?

Secondary pneumonias develop secondary to various pathological processes and systemic conditions. Most frequently, they complicate:

Chronic bronchopulmonary diseases.
Various systemic or somatic diseases.
Other infections where the primary focus is outside the lungs (for example, severe forms of influenza are frequently complicated by secondary bacterial pneumonia).

Additionally, all cases of hospital-acquired (nosocomial) acute pneumonias are classified as secondary.

What are the main pathogens typical of nosocomial pneumonia?

Hospital-acquired infections are usually caused by Gram-negative aerobic bacteria with mixed flora. These include Klebsiella, Pseudomonas aeruginosa, Escherichia coli, Proteus, staphylococci, and streptococci.

What is the difference between primary and secondary pneumonia?

Primary pneumonias occur as independent diseases in previously healthy individuals. Secondary pneumonias develop against the background of chronic bronchial pathology, systemic diseases, or other infections.

What is interstitial pneumonia and what are its alternative names?

It is an inflammation affecting the alveolar wall and interstitial tissue with secondary exudate accumulation in the alveoli. Synonyms for this condition include acute pneumonitis and atypical pneumonia.

Go deeper

Etiological features and mechanisms of lung parenchyma damage by viruses and pyogenic bacteria
Risk factors for acute pneumonia, including smoking, immunodeficiencies, and airway obstruction
Classification of bronchopneumonia according to the extent of the process
Timeframes and division of nosocomial pneumonia based on the timing of onset in the hospital setting
Epidemiological morbidity and mortality indicators in international populations