The emergence of highly pathogenic avian influenza viruses has received much attention due to the severe consequences of their occasional spread to humans, as well as the large toll they take on the poultry industry. Here we argue that the main barriers to the emergence of these viruses are imposed by immunity to related strains rather than the ability of the virus to acquire the necessary mutations. We show that, under these circumstances, patterns of influenza in different avian species are strongly dependent on their lifespan and that processes that alter the interactions between species of different lifespans—such as changes in farming practices—could promote the emergence of highly pathogenic types.

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