Origins of the flu:
According to Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis, all influenza viruses originate in wild birds.
Pandemic variants often occur when a bird or avian virus infects pigs. The virus may then recombine, or mutate, to form a new virus that is able to infect humans and possibly pass from person to person.
The disease we consider the normal flu, which tends to hit America every winter, is left over from the 1968 pandemic.
Mutations
There are two main ways the flu virus mutates or evolves: antigenic shift and antigenic drift.
Antigenic shift is when two different strains of the flu combine to form a subtype, or a combination of the two originals. This only occurs in type A influenza, because it affects multiple species (birds, pigs and humans, for example).
Antigenic drift is the change of a virus over time as it tries to evade the immune system of the organism it is infecting. This is why a new flu vaccine has to be prepared each year.
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