Influenza is an acute respiratory illness caused by influenza A or B viruses that occurs in outbreaks and epidemics worldwide, mainly during the winter season. Signs and symptoms of upper and/or lower respiratory tract involvement are present, along with indications of systemic illness such as fever, headache, myalgia, and weakness. Although acutely debilitating, influenza is a self-limited infection in the general population (uncomplicated influenza); however, it is associated with increased morbidity and mortality in certain high-risk populations (complicated influenza).
The United States Centers for Disease Control and Prevention (CDC), in collaboration with the World Health Organization and its reporting network, tracks influenza virus isolates throughout the world to monitor disease activity and to predict the appropriate components for the annual influenza vaccine. Information, which is updated weekly, is available at the CDC web site.
The clinical manifestations of seasonal influenza in adults will be reviewed here. The clinical manifestations of this infection in children are discussed separately. The epidemiology, diagnosis, prevention, and treatment of seasonal influenza infection, as well as the clinical manifestations of pandemic H1N1 influenza ("swine influenza") and avian influenza, are reviewed elsewhere. (See "Seasonal influenza in children: Clinical features and diagnosis" and "Epidemiology of influenza" and "Diagnosis of seasonal influenza in adults" and "Seasonal influenza vaccination in adults" and "Prevention of seasonal influenza in adults" and "Infection control measures to prevent seasonal influenza in healthcare settings" and "Clinical manifestations and diagnosis of pandemic H1N1 influenza ('swine influenza')" and "Treatment of seasonal influenza in adults" and "Clinical manifestations and diagnosis of avian influenza".)
Large amounts of influenza virus are often present in respiratory secretions of infected persons. As a result, infection can be transmitted through sneezing and coughing; influenza is thought to be transmitted primarily via large particle droplets (>5 microns) [1,2].
Acquisition via large particle droplets requires close contact with an infected individual, since large particles do not remain suspended in the air and travel only short distances (~6 feet) . Limited data suggest that small particle respiratory droplets, which become aerosolized and stay suspended in the air for long periods, also contain influenza virus [4-7]. However, the contribution of small particles to airborne transmission of influenza is unclear. Contact with surfaces that have been contaminated with respiratory droplets is another potential source of transmission, but is not a well established mode of transmission. Although the respiratory tract is presumed to be the primary site for the acquisition of infection, one study has suggested that transocular entry of influenza virus can occur .