As far as I know the virus H1N1 itself didn't really evolve in the sense that it got milder - all hosts either developed immunity or died off. Now our immune systems encounter one of the endemic, seasonal strains of H1N1 when we are young (or we get a flu vaccine) - however from time to time a new strain of H1N1 emerges that is dissimilar enough, see e.g. 1977 flu or the 2009 flu outbreak.