Influenza is an acute systemic viral infection that primarily affects the respiratory tract and carries a significant mortality. It is caused by influenza A virus or, in milder form, influenza B virus. Infection is seasonal, and variation in the haemagglutinin (H) and neuraminidase (N) glycoproteins on the surface of the virus leads to disease of variable intensity each year. Minor
changes in haemagglutinin are known as ‘genetic drift’, whereas a switch in the haemagglutinin or neuraminidase antigen is termed ‘genetic shift’. Nomenclature of influenza strains is based on these glycoproteins, e.g. H1N1, H3N2 etc. Genetic shift results
in the circulation of a new influenza strain within a community to which few people are immune, potentially initiating an influenza epidemic or pandemic in which there is a high attack rate and there may be increased disease severity.

Clinical features
After an incubation period of 1–3 days, uncomplicated disease leads to fever, malaise and cough. Viral pneumonia may occur, although pulmonary complications are most often due to
superinfection with Strep. pneumoniae, Staph. aureus or other bacteria. Rare extrapulmonary manifestations include myositis, myocarditis, pericarditis and neurological complications (Reye’s syndrome in children, encephalitis or transverse myelitis). Mortality is greatest in the elderly, those with medical comorbidities and pregnant women. Polymorphisms in the gene encoding an antiviral protein, interferon-induced transmembrane protein 3 (IFITM3), are associated with more severe influenza

Diagnosis
Acute infection is diagnosed by viral antigen or RNA detection in a nasopharyngeal sample. The disease may also be diagnosed retrospectively by serology.