The H1N1 pandemic in 2009 was a mixture between Human and Avian forms of the influenza virus. How did breaking the species barrier lead to development of this virus?
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Antgenic shift leads to the generation of new types of Influenza A. Reassortment of the RNA segments (8 per viral particle) between two viruses that have coinfected the same host can lead to the creation of a new type of influenza virus, depending on whether or not the reassortment results in a viable virus. Humans, birds, and other animals are infected by different types of influenza. If the viruses reassort in a bird in such a way that RNA segments that allow infection in both birds and humans (reassortment of genes from avian and human viruses in the same host), the result will be capable of infecting birds and humans. This is how new types of influenza A viruses arise. Antigenic drift is also a factor, but shift is primarily responsible for the generation of H1N1 in this context.