This video describes the process by which viruses can share a segment of their gemone with another strain during infection.
the influenza Type-A virus is capable of what is known as antigenic shift. The current flu in circulation is an entirely new, mutated pathogen formed from elements of human, pig, and avian virus strains.
A sudden shift in the antigenicity of a virus resulting from the recombination of the genomes of two viral strains. Antigenic shift is seen only with influenza A viruses. It results usually from the replacement of the hemagglutinin (the viral attachment protein that also mediates the entry of the virus into the cell) with a novel subtype that has not been present in human influenzaviruses for a long time. The source of these new genes is the large reservoir of influenzaviruses in waterfowl. The consequences of the introduction of a new hemagglutinin into human viruses is usually a pandemic, or a worldwide epidemic.