Science Forums 2

[mynah]

Perhaps - but it would be very hard to either prove or disprove that H1N1 circulated in humans before 1918. The virulence of the disease suggests a species jump - although the partial immunity of some populations may be due to exposure to not greatly dissimilar strains. As it were, H1N1 never caused a severe epidemic again: Even the Asian flu epidemic of 1957 that involved a double antigenic shift to H2N2 was not as serious, nor was the Hong Kong flu epidemic of 1968 (single antigenic shift to H3N2). The avian flu outbreak that started in Asia in 1997 was caused by a species jump. H5N1 had been identified in birds decades before, but had never been seen in humans until then. Unlike the Spanish flu it was marked by low infectivity but very high mortality in those affected - a typical situation in the early stages of a species jump.