Influenza type A viruses are classified into subtypes based on their two surface 
proteins, hemagglutinin (HA) and neuraminidase (NA). The HA protein facilitates 
the viral binding and entering a host cell and the NA protein helps the release 
of viral progeny from the infected cell. The complementary roles of HA and NA 
entail their collaboration, which has important implications for viral 
replication and fitness. The HA protein from early strains of pandemic 2009 H1N1 
of swine origin preferentially binds to human type receptors with a weak binding 
to avian type receptors. This virus caused several human deaths in December 2013 
in Texas, USA, which motivated us to investigate the changes of genetic features 
that might contribute to the surged virulence of the virus. Our time series 
analysis on the strains of this virus collected from 2009 to 2013 implied that 
the HA binding preference of this virus in USA, Europe, and Asia has been the 
characteristic of swine H1N1 virus since 2009. However, its characteristic of 
seasonal human H1N1 and its binding avidity for avian type receptors both were 
on steady rise and had a clear increase in 2013 with American strains having the 
sharpest surge. The first change could enhance the viral transmission and 
replication in humans and the second could increase its ability to cause 
infection deep in lungs, which might account for the recent human deaths in 
Texas. In light of HA and NA coadaptation and evolutionary interactions, we also 
explored the NA activity of this virus to reveal the functional balance between 
HA and NA during the course of virus evolution. Finally we identified amino acid 
substitutions in HA and NA of the virus that were critical for the observed 
evolution.