Whenever a new version of virus evolves , there's a chance that it'll become pandemic. Its because human does not have immuno resistance against that new strain. However, the interesting Ques. is how these new strains evolve?
New strain of virus ( like " Nobel H1N1" ) can be emerged via two basic processes.They r -
1. Antigenic Shift
2. Antigenic Drift
Antigenic shift is the process by which at least two different strains of a virus (or different viruses), especially influenza, combine to form a new subtype having a mixture of the surface antigens of the two original strains. The term antigenic shiftis more often applied specifically (but is not limited) to the influenza. Antigenetic shift is a specific case of reassortment or viral shift that confers a phenotypic change.
Genomes of influenza virus consist of 8 distinct segments of RNA . These segments act like mini-chromosomes, and each time a flu virus is assembled, it requires one copy of each segment. If a single host (a human, a chicken, or other animal) is infected by two different strains of the influenza virus, then it is possible that new assembled viral particles will be created from segments whose origin is mixed, some coming from one strain and some coming from another. The new reassortant strain will share properties of both of its parental lineages.
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Another term Antigenic drift is completely diferent and should not be confused with Antigenic shift. Random mutations in the genes of a virus drives antigenic drift that changes the antigens of the virus. As these changes accumulate it may help the virus to evade the immune system since antigens are what the immune system recognizes.
Thus a new strain can evolve by the genetic changes in viral genome... !
Reference:
http://en.wikipedia.org/wiki/Antigenic_drift
http://en.wikipedia.org/wiki/Antigenic_shift
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