Influenza is an acute febrile illness caused by influenza A or B viruses. It occurs mainly in winter in temperate climates, and throughout the year in tropical Australia. It is highly contagious and of considerable public health concern because of the rapidity with which epidemics evolve and the associated morbidity and mortality.
Most influenza illnesses resolve over about 1 week without specific medical intervention.
People at particular risk for complicated infection are those > 65 or < 5 years old, those with chronic medical comorbidities, residents of chronic care facilities (including nursing homes), and women in the second or third trimester of pregnancy.
Complicated influenza infection most commonly manifests as primary viral pneumonia, combined viral and bacterial pneumonia, and secondary bacterial pneumonia.
Rare but serious complications of influenza include central nervous system involvement (eg, encephalitis, transverse myelitis, aseptic meningitis, and Guillain–Barré syndrome).
The recent emergence of avian influenza A/H5N1 and confirmation of sporadic cases of human H5N1 infection have heightened concern about an impending human influenza pandemic, either from a human form of H5N1 or a primary new human influenza strain.
H5N1 infection in humans has been associated with severe illness and a > 50% mortality rate, with high mortality in people aged 10–39 years.
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