Introduction
Influenza A virus belongs to a family of RNA viruses termed the
orthomyxoviridae. It is the main cause of seasonal or pandemic flu,
an infectious disease characterized by high morbidity and significant
mortality. Since influenza A virus is under continuous evolvement
due to antigenic mutation, adaptation, and reassortment, highly
virulent strains may appear unexpectedly, resulting in epidemics
locally or pandemics worldwide such as the 1918 H1N1 (Spanish
flu), 1957 H2N2 (Asian flu), 1968 H3N2 (Hong Kong flu), 2005
H5N1 (bird flu), 2009 H1N1 (swine flu), and more recently H7N9
(bird flu) of 2013. Influenza A virus pandemic may cause significant
social health crisis and loss of life. For example, the pandemic of
“Spanish flu” (H1N1) in 1918–1919 alone caused at least 20 million
deaths. Besides, seasonal flu affects about 20% world population
and causes 250,000 to 500,000 deaths per year based on a recent
study1