like evolution .. one strains binds with other things and you get a new strain. Like when humans breed with other types of people.. YOu get + and - of both genes from the parents. Out pops a new person that is different.
Major evolutionary changes (emergence of new viruses) in the influenza type A viruses occur by the mixing or re-assorting of their genetic material causing changes in their external surface HA and NA antigens. This phenomenon is known as “antigenic shift.” The genetic material or genomes of influenza viruses occur in eight separate molecules or segments. If two different subtypes of influenza A virus infect the same cell, their genetic segments are able to reassort and produce a new influenza virus with segments from both infecting viruses. As an example, if a H3N5 virus and a H2N2 virus infect the same cell the following offspring viruses can be produced: H3N5, H2N2, H3N2, and H2N5. This re- assortment can occur between human and animal isolates. Therefore, new viruses can be produced which can replicate in humans, but have new subtypes of animal HA and NA antigens. There are no protective antibodies to these antigens in the human population, so the new virus can spread very rapidly around the world.