Viral pneumonia
- Lesion
Viruses inflame the alveolar walls rather than filling the alveoli. Most cases are mild; influenza and coronaviruses can cause diffuse alveolar damage.
What it is
Community-acquired viral pneumonia is caused mainly by influenza A and B, respiratory syncytial virus, human metapneumovirus, parainfluenza, adenovirus and coronaviruses including SARS-CoV-2. Most share one lesion: inflammation of the alveolar septa with lymphocytes and macrophages, often called atypical or interstitial pneumonia. Severe cases develop diffuse alveolar damage, and secondary bacterial infection is a main cause of death.
Structure
- Gross: patchy or lobar red-blue, congested areas; pleural effusions uncommon.
- H&E: mononuclear infiltrate in widened alveolar septa; little intra-alveolar exudate unless severe.
- Severe: hyaline membranes, type II hyperplasia, squamous metaplasia of bronchioles; microthrombi in COVID-19.
- Bacterial superinfection adds neutrophils, ulcerative bronchitis and bronchopneumonia.
Step by step
- Viruses bind receptors on respiratory epithelium (hemagglutinin to sialic acid for influenza, spike to ACE2 for SARS-CoV-2).
- They replicate in and kill ciliated and alveolar epithelial cells.
- Interferons and cytokines bring lymphocytes and macrophages into the septa.
- Loss of cilia and epithelium lets bacteria settle: secondary pneumonia (S. aureus, pneumococcus).
- In severe cases an excessive cytokine response and endothelial damage produce diffuse alveolar damage.
Bacterial vs viral (interstitial) pneumonia
| Feature | Bacterial | Viral (interstitial) |
|---|---|---|
| Where inflammation sits | Inside alveoli | In alveolar walls |
| Main cells | Neutrophils | Lymphocytes, macrophages |
| Gross pattern | Lobar or bronchopneumonia | Patchy, diffuse |
| Sputum | Purulent, organisms on Gram stain | Scant, no organisms |
| Severe complication | Abscess, empyema | Diffuse alveolar damage, bacterial superinfection |
Clinical notes
- Fever, dry cough, myalgia and bilateral interstitial infiltrates, often milder on examination than on imaging.
- High-risk groups: elderly, pregnant, chronic disease, immunosuppression; yearly influenza vaccination reduces deaths.
- Late worsening after initial improvement suggests bacterial superinfection.
Test yourself
Think of the answer, then tap the question to see it.
Where is the inflammation in viral pneumonia, compared with bacterial pneumonia?
Mainly in the alveolar septa (interstitium) with lymphocytes and macrophages, rather than filling the alveoli with neutrophils.
What is the difference between antigenic drift and shift in influenza?
Drift is gradual point mutation causing yearly epidemics; shift is reassortment of whole genome segments producing a new subtype and pandemics.
Through which receptor does SARS-CoV-2 enter respiratory cells?
ACE2, bound by the viral spike protein.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Flu (MedlinePlus, US National Library of Medicine, 2025); Pneumonia (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.