The Swine influenza a tendency to keep changing its form. This is actually its own way to dodge the protective antibodies in Swines, avians and humans that have developed in response to previous exposures to influenza or to influenza vaccines.The virus undergoes an alteration after every one or two years. But at an interval of about a decade, when a large part of the world's population has developed some level of resistance to these minor changes, it changes itself drastically to easily infect the masses around the world, often infecting hundreds of millions of people who have lost their antibody resistance . Definitely a smart species, I must say. There have been instances when the influenza virus has changed its form over a much shorter period. For instance, during the Spanish flu pandemic, the initial wave of the disease was relatively mild, while the second wave of the disease a year later was highly lethal.
During the 1918 pandemic, when pigs became sick at the same time as humans, it was deduced that Swine flu is the same as human influenza, atleast the similarities were quite a lot. But only about ten years later, i.e. in 1930, a virus was identified as a cause of disease in pigs. For the next 60 years, Swine influenza strains meant exclusively H1N1. It was only between 1997 and 2002, that novel viruses of three different subtypes and five genotypes entered the scene as the carriers of influenza among pigs and then the following years brought the H3N2 strains. These strains, which include genes derived by from human, swine and avian viruses, have become a major cause of swine influenza in North America. Reassortment between H1N1 and H3N2 produced H1N2.
As well as persisting in pigs, the descendants of the 1918 virus have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza. However, there have been very less cases of viral transfer from pigs to humans.
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