The 1918 Swine Flu outbreak
The SIV outbreak of the years 1918-1919 had an impact that was not limited to the years during which it occurred. Every influenza pandemic that has occurred since that time, in fact - nearly all cases of SIV worldwide, have been caused by the descendants of the 1918 virus; to include the H1N1, H2N2, and H3N2 viruses. The latter viruses are composed of key genes from the 1918 virus which subsequently incorporated avian flu genes, making the 1918 virus the, 'mother of all pandemics.'
Even though there were both clinical and epidemiological similarities to the influenza pandemics of 1889, 1847, and earlier pandemics, many people questioned where this explosively fatal disease of 1918 could even be influenza at all. The question did not even begin to reach some form of resolution until the 1930's when closely related flu viruses were isolated. Studies known as, 'Seroepidemiologic,' studies linked viruses to the 1918 pandemic, and subsequent studies indicate that descendants of the 1918 virus still persist in pigs. These viruses most likely still circulate in human beings as well, undergoing gradual antigenic shifts which cause annual epidemics. H1N1 viruses, descended from the 1918 strain, along with H3N2 viruses, have been co-circulating around the world for decades, showing little evidence of imminent extinction.
Both before and after 1918, the majority of flu pandemics developed in Asia and spread from there to the rest of the world. The pandemic of 1918 spread in three distinct waves over a more-or-less twelve month period of time in Asia, Europe, and North America. Neither the historical or epidemiological data provide adequate information to identify the exact geographical origin of the virus that caused this pandemic. The next outbreak of swine flu occurred in 1976