[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Lung](https://durgical.com/en/general-pathology/areas/lung/) › Viral pneumonia

# 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.

Updated Nov 12, 2026

## Key points

- Interstitial pattern: widened, edematous septa with lymphocytes, macrophages and plasma cells; alveoli may be clear.
- Severe disease: hyaline membranes (DAD), as in severe influenza and COVID-19.
- Influenza: antigenic drift causes yearly epidemics; antigenic shift (reassortment, often via pigs or birds) causes pandemics.
- SARS-CoV-2 enters cells through ACE2; severe COVID-19 shows DAD, microthrombi and endothelial injury.
- Human metapneumovirus resembles RSV: bronchiolitis and pneumonia in young children, the elderly and the immunocompromised.
- Cytopathic clues: CMV owl's-eye inclusions, herpes and adenovirus necrosis, measles giant cells.

## 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](https://durgical.com/en/conditions/community-acquired-pneumonia/) | Inside alveoli | In alveolar walls |
| [Main cells](https://durgical.com/en/conditions/influenza/) | 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.

## Memory aid

Bacteria fill the rooms (alveoli); viruses inflame the walls (septa).

## 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.

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## Connections

## Related 3

- [Diffuse alveolar damage (ARDS)](https://durgical.com/en/general-pathology/diffuse-alveolar-damage/)
- [Bacterial pneumonia: lobar vs bronchopneumonia](https://durgical.com/en/general-pathology/bacterial-pneumonia-lobar-vs-bronchopneumonia/)
- [Community-acquired pneumonia: organisms](https://durgical.com/en/general-pathology/community-acquired-pneumonia-organisms/)

## Related conditions 5

- [Influenza (flu)](https://durgical.com/en/conditions/influenza/)
- [COVID-19](https://durgical.com/en/conditions/covid-19/)
- [Bronchiolitis (RSV)](https://durgical.com/en/conditions/bronchiolitis/)
- [Measles](https://durgical.com/en/conditions/measles/)
- [Hantavirus pulmonary syndrome](https://durgical.com/en/conditions/hantavirus-pulmonary-syndrome/)

## Microorganisms 4

- [Influenza virus](https://durgical.com/en/microbiology/influenza-virus/)
- [Human metapneumovirus](https://durgical.com/en/microbiology/human-metapneumovirus/)
- [SARS-CoV-2](https://durgical.com/en/microbiology/sars-cov-2/)
- [Respiratory syncytial virus](https://durgical.com/en/microbiology/respiratory-syncytial-virus/)

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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](https://medlineplus.gov/influenza.html) (MedlinePlus, US National Library of Medicine, 2025); [Pneumonia](https://medlineplus.gov/pneumonia.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/viral-pneumonia/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/viral-pneumonia/