Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms (e.g. fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis).

Among children, otitis media, nausea, and vomiting are also commonly reported with influenza illness.

Influenza illness typically resolves after a limited number of days for the majority of persons, although cough and malaise can persist for more than 2 weeks.

Among certain persons, influenza can exacerbate underlying medical conditions (e.g. pulmonary or cardiac disease), lead to secondary bacterial pneumonia or primary influenza viral pneumonia, or occur as part of a co-infection with other viral or bacterial pathogens.

Young children with influenza infection can have initial symptoms mimicking bacterial sepsis with high fevers, and <= 20% of children hospitalized with influenza can have 2 febrile seizures.

Influenza infection has also been associated with encephalopathy, transverse myelitis, Reye syndrome, myositis, myocarditis, and pericarditis.