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Abstract  The current pandemic threat can be best understood within an ecological framework that takes account of the history of past pandemics caused by influenza A, the relationships between pandemic and seasonal spread of influenza viruses, and the importance of immunity and behavioural responses in human populations. Isolated populations without recent exposure to seasonal influenza seem more susceptible to new pandemic viruses, and much collateral evidence suggests that this is due to immunity directed against epitopes shared between pandemic and previously circulating strains of inter‐pandemic influenza A virus. In the highly connected modern world, most populations are regularly exposed to non‐pandemic viruses, which can even boost immunity without causing influenza symptoms. Such naturally‐induced immunity helps to explain the low attack‐rates of seasonal influenza, as well as the …