"Like the 1918 virus [Spanish flu], H5N1 influenza has unusually high virulence and can capitalize on an absence of preexisting immunity in humans, at least in certain age groups. Although the two viruses differ in their transmissibility among humans, there is concern that currently circulating H5N1 viruses will evolve into a pandemic strain by adapting to humans through genetic mutation or reassortment with human influenza strains ..." (Quoted from paper)