Influenza
An acute respiratory viral infection whose rapid antigenic change allows it to repeatedly evade population immunity, producing seasonal epidemics and occasional pandemics.
Definition
Influenza is an acute respiratory infection caused mainly by influenza A or B viruses. It is often self-limiting but can cause viral pneumonitis, secondary bacterial infection, decompensation of chronic disease and severe systemic illness.
First principles
Influenza infects and destroys the respiratory epithelium directly
The virus binds sialic acid receptors on respiratory epithelial cells via haemagglutinin, replicates within them and causes direct cytopathic cell death, stripping away the ciliated epithelium that normally clears secretions and pathogens. This explains both the abrupt onset of systemic symptoms, from the intense innate immune and cytokine response to rapid viral replication, and the vulnerability to secondary bacterial pneumonia once the protective epithelial barrier is destroyed.
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