With the appearance of a new H2N2 pandemic strain in 1957 (“Asian flu”), the direct H1N1 viral descendants of the 1918 pandemic strain disappeared from human circulation entirely, although the related lineage persisted enzootically in pigs. But in 1977, human H1N1 viruses suddenly “reemerged” from a laboratory freezer (9). They continue to circulate endemically and epidemically.