Genetic evolution of influenza (flu) virus. Artwork showing how reassortment of the influenza virus genome led to the evolution of the H7N9 virus in China, which emerged in the human population in March 2013. Reassortment occurs when two or more influenza viruses co-infect a single host and exchange genes. The virus' genome is split into 8 segments. Two of the segments (HA and NA) code for the envelope proteins haemagglutinin (H) and neuraminidase (N), which determine the strain of virus. It is thought that multiple reassortment events, in habitats shared by wild and domestic birds, and/or in live bird markets resulted in the H7N9 strain.