[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Respiratory RNA viruses](https://durgical.com/en/microbiology/groups/respiratory-rna-viruses/) › Influenza virus

[Microbiology](https://durgical.com/en/microbiology/) · [Respiratory RNA viruses](https://durgical.com/en/microbiology/groups/respiratory-rna-viruses/)

# Influenza virus

- Virus
- Enveloped
- Droplet
- Exam priority: high

The flu virus. It causes sudden fever, cough and muscle aches, and it changes a little every year, which is why the vaccine is repeated.

Updated 2026-11-01

Also called: flu virus, influenza A and B

## Key points

- Negative-sense orthomyxovirus with eight segments
- Drift = yearly epidemics; shift (reassortment) = pandemics, influenza A only
- HA binds sialic acid; NA releases virions and is the oseltamivir target
- Staphylococcus aureus pneumonia after influenza
- Droplet precautions

## At a glance

**Shape**
Spherical or filamentous (pleomorphic)

**Genome**
Single-stranded, negative-sense RNA in eight segments

**Envelope**
Enveloped

**Family**
Orthomyxoviridae, genera Alphainfluenzavirus and Betainfluenzavirus

**Size**
About 80 to 120 nm

## Morphology

- Envelope with spikes of haemagglutinin (HA, binds sialic acid) and neuraminidase (NA, releases virions)
- Matrix protein M2, an ion channel in influenza A
- Influenza A is subtyped by HA and NA, for example A(H1N1) and A(H3N2); B is split into lineages

## Transmission

- Respiratory droplets from coughs or sneezes
- Contaminated hands and surfaces
- Aerosols during aerosol-generating procedures
- Incubation 1 to 4 days; contagious from a day before symptoms

**Reservoir:** Humans (A and B); Wild waterfowl and pigs (influenza A), the source of new viruses

### Isolation precautions

What the care team adds to standard precautions.

**Droplet**
Surgical mask within about a metre; single room.

## Virulence factors

- Antigenic drift: point mutations in HA and NA that cause yearly epidemics
- Antigenic shift: reassortment of segments between human and animal viruses (influenza A only) that can cause pandemics
- Destruction of ciliated epithelium, which opens the way to bacterial superinfection

## Diseases it causes

- Seasonal and pandemic influenza [Influenza (flu) →](https://durgical.com/en/conditions/influenza/)
- Primary viral pneumonia or secondary bacterial pneumonia (Staphylococcus aureus, pneumococcus) [Community-acquired pneumonia →](https://durgical.com/en/conditions/community-acquired-pneumonia/)

## Clinical picture

- Sudden high fever, headache, severe myalgia, dry cough, sore throat, malaise
- Complications: viral or secondary bacterial pneumonia, myositis, myocarditis, worsening of chronic disease
- Higher risk: pregnancy, children under 5, adults over 65, chronic disease and immunosuppression
- Reye syndrome in children given aspirin

## Diagnosis

- RT-PCR of a nasopharyngeal swab (reference test)
- Rapid antigen tests, with limited sensitivity
- Clinical diagnosis during high circulation

## Treatment

Neuraminidase inhibitors (oseltamivir, zanamivir) and the cap-dependent endonuclease inhibitor (baloxavir marboxil), most useful when started early in people at risk or with severe illness. Adamantanes are no longer recommended because of widespread resistance.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

- [Oseltamivir](https://durgical.com/en/pharmacology/oseltamivir/)
- [Zanamivir](https://durgical.com/en/pharmacology/zanamivir/)
- [Baloxavir marboxil](https://durgical.com/en/pharmacology/baloxavir-marboxil/)

## Prevention

- Yearly vaccine, because strains change
- Hand hygiene and cough etiquette
- Droplet precautions in hospital
- Antiviral chemoprophylaxis in selected situations

**Vaccine:** Yearly inactivated vaccine; Colombia's PAI offers it to young children, pregnant women, older adults and people with chronic disease.

## In Colombia

In Colombia influenza circulates year-round with peaks tied to the rainy seasons; the INS monitors severe acute respiratory infection (SARI, «IRAG») and circulating viruses through a sentinel surveillance network.

## Memory aid

Flu "changes its make-up" every year (drift) and now and then "changes its whole outfit" (shift): that is why the vaccine is renewed.

## Test yourself

Think of the answer, then tap the question to see it.

## How do antigenic drift and shift differ in influenza?

Drift is small mutations in HA and NA that cause yearly epidemics; shift is reassortment of segments between different viruses that can cause pandemics.

## Which viral enzyme does oseltamivir block?

Neuraminidase, which releases new virions from the infected cell.

## Which bacterium is linked to severe pneumonia after influenza?

Staphylococcus aureus, as well as pneumococcus.

## Which isolation precaution applies to a patient with influenza?

Droplet precautions on top of standard ones: a surgical mask at close range.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/influenza-virus/)

## Related

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

## Related conditions

- [Influenza (flu)](https://durgical.com/en/conditions/influenza/)
- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

**Contact**
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

**Droplet**
Surgical mask within about a metre; single room.

**Airborne**
N95 respirator and a negative-pressure room.

[← Back to the group: Respiratory RNA viruses](https://durgical.com/en/microbiology/groups/respiratory-rna-viruses/)

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Influenza (seasonal)](https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)) (World Health Organization (WHO), 2025); [Appendix A: Type and Duration of Precautions Recommended for Selected Infections and Conditions](https://www.cdc.gov/infection-control/hcp/isolation-precautions/appendix-a-type-duration.html) (CDC, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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Source: https://durgical.com/en/microbiology/influenza-virus/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/influenza-virus/