The genetic robustness of influenza viruses and the accompanying structure of neutral genetic networks probably arose to foster the epidemiological spread of the virus in humans and/or other animal hosts. The size of these neutral networks is of consequence in the evolution of influenza, as suggested by modern robustness theory. 8 A HA gene with relatively low robustness would feature a small neutral network; this characteristic might cause the virus to molecularly evolve rather quickly, perhaps outpacing the availability of susceptible individuals in a host population, slowing or halting spread of the virus. But a relatively larger neutral network would afford greater robustness of HA genes in influenza, affording long-term persistence of the virus as it drifts neutrally through sequence space before fortuitously achieving a genotype with a beneficial novel surface antigen that promotes reemergence.