Chinese guidelines for diagnosis and treatment of influenza (2011).

Influenza, an acute respiratory infection caused by influenza virus, is one of public concerns to human health. Epidemiologically, influenza is characterized by a sudden outbreak and rapid transmission with varying epidemics, seasonality, and a high morbidity yet low case fatality (usually 0.003% to 0.03% except for human avian flu). The past 300 years has witnessed at least six global pandemics, including four in the 20th century -- three of them originated in China.