The second theory is the most likely explanation for the 1918 pandemic virus. The third theory has been advanced to explain the reappearance of H1N1 virus in 1977 that resembled virus from 1950, although it is not currently understood where and how any influenza virus could remain unrecognized for many years. A virus has shown the ability to cause pandemics. Indeed, the main difference between these two viruses is the substitution of the gene coding for the haemagglutinin in 1968, changing it from H2 to H3. Due to the segmented nature of the influenza viral genome, genetic reassortment readily occurs during mixed infections.
China has a large population and many communities practice both pig and duck farming.
Also, there is wide climatic variation from North China to South China, so that influenza infections of humans occur normally every month of the year somewhere in this single country. This combination of factors could be the key to the origin of influenza pandemics. Thus, it is possible that the agricultural practices and ecological circumstances in this area provide continual opportunities for the co-infection of animals to occur with human, avian and swine influenza viruses. Such co-infections would enable reassortants to arise, from which those with human epidemic properties could then be selected through a series of transmissions between animals or humans over an extended period of time. If this were so, then the combination of evidence again would suggest an importance for avian species as a reservoir of influenza virus genes capable of contributing to human pandemic strains.