Introduction of new influenza type-A viruses, carrying different combinations of the viral envelope glycoproteins haemagglutinin (H) and neuraminidase (N), have led to three major pandemics of influenza in humans this century. Phylogenetic evidence suggests that these viruses have originated from avian influenza A viruses, either unchanged or after reassortment with humaninfluenza A viruses. In aquatic birds, all of the known H and N antigenic varieties (15 varieties carry H, nine carry N envelope glycoproteins) apparently circulate in a genetically conserved fashion. Viruses carrying the H1N1, H2N2 and H3N2 combinations were responsible for the Spanish flu of 1918, the Asian flu in 1957 and Hong Kong flu in 1968, respectively. An influenza A virus of the H5N1 subtype has now been identified in a human patient, raising discussions about its potential to spark a new human influenza pandemic.