The clinical spectrum of influenza ranges from asymptomatic infection to primary viral pneumonia that may progress to death.
Patients presenting with influenza-like illness (ie temperature of 37.8ºC, cough and/or sore throat and absence of a known cause other than influenza) might be infected with different types of influenza virus [eg avian influenza (H5N1)] as well as other respiratory pathogens.
A high index of suspicion is needed to recognize influenza in hospitalized patients.
Pneumonia is the most common complication of influenza virus.