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Microbiology · 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

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

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:

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.

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.

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← Back to the group: 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) (World Health Organization (WHO), 2025); Appendix A: Type and Duration of Precautions Recommended for Selected Infections and Conditions (CDC, 2024).

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Representative model, for educational purposes only.