Pandemic viruses emerge as a result of a process called "antigenic shift," which causes an abrupt or sudden, major change in influenza a viruses. as billions of viruses replicate, mutations can cause variation in infectivity. these changes are caused by

The answer would be: new combinations of the HA and/or NA proteins on the surface of the virus during viral evolution

Antigenic shift is a mutation in the antigen that causes its shape changed. This change is dangerous because the white blood cells will not recognize them as the old virus, make the strong immune response delayed. This changes mostly caused by a random mutation.