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.
