Influenza
An acute respiratory viral infection whose rapid antigenic change allows it to repeatedly evade population immunity, producing seasonal epidemics and occasional pandemics.
In a nutshell
Influenza directly destroys respiratory epithelial cells, stripping the mucociliary barrier and triggering a vigorous cytokine response responsible for the abrupt systemic symptoms. Constant antigenic drift erodes immunity year on year, while occasional antigenic shift produces pandemic strains against which there is little pre-existing immunity.
Classic presentation
Abrupt-onset high fever, myalgia, headache and dry cough with marked prostration, developing much more suddenly than a common cold.
Key points
- Direct cytopathic destruction of respiratory epithelium removes the mucociliary defence, predisposing to secondary bacterial pneumonia.
- Antigenic drift causes gradual immune evasion requiring annual vaccine reformulation; antigenic shift causes sudden pandemic strains via genome reassortment.
- The abrupt onset and severity of systemic symptoms (not just respiratory symptoms) distinguish influenza from the common cold.
- Neuraminidase inhibitors are only effective if started within 48 hours of symptom onset and are targeted at those at risk of severe disease.
- Vaccination is the primary preventive strategy but must be updated annually because of antigenic drift.
First-line investigation
Respiratory viral PCR from a nasopharyngeal or throat swab to confirm influenza and distinguish it from other respiratory viruses.
First-line management
Supportive care for most patients; a neuraminidase inhibitor within 48 hours of onset for those at risk of severe disease.
Exam traps
- Oseltamivir given after 48 hours of symptom onset has little benefit: timing is the key discriminator in exam stems.
- Worsening symptoms after initial improvement suggests secondary bacterial pneumonia, not a normal course of viral illness.
- Influenza vaccination does not guarantee protection in a given season because of antigenic drift, so vaccinated patients can still present with influenza.
- A gradual onset with predominantly nasal symptoms points away from influenza and towards a common cold.
Educational content pending clinical review. Not medical advice.