Influenza is a disease known for a long time. But in the recent decades its newer and mutant forms such as the H1N1, H3N1 or the swine flu, avian flu as they are popularly called are seen to be spreading in epidemic and pandemic proportions in most regions of the world. All types of influenza are basically contagious respiratory diseases

caused by the Orthomyxoviridae RNA family of viruses whose strains A, B and C are of significance to human beings. They spread mostly through the air and enter the mouth and nose (WHO, 2010). Secondary animal hosts such as birds and pigs and constant evolutionary strains caused by rapid mutations (especially in the A type) are the most prominent characteristics of these viruses. An example of this scourge is the pandemic of the 2009 H1N1 outbreak that spread around the world from North America.

The main features of the orthomyxoviruses is a typical and common structural design, that consists of an inner core made up of 8 stranded and segmented RNA genome that forms the ribonucleoprotein complex in a molecular nucleoprotein and polymerase protein matrix. The glycoprotein molecules form a lipid bilayer membrane covering this. The surface proteins are further distinguished into the spikes that are made of the rectangular hemagglutinin HA and the mushroom-shaped neuraminidase NA (see fig.1).

The types of viruses are classified by the mutations that occur in these surface proteins thus producing more strains of the virus. Examples of such strains are H1N1, H2N2 and H3N2. The recombination and re-assortment of genes among the different forms results in the formation of still newer strains of the virus, for example, the H2N2 virus combining with a H3N1 could produce H3N2 type. The classification is further distinguished with the geographical region of its origin, the sequential number and the year it was isolated.

Because of the rapid mutations that are constantly occurring in these viruses, new strains of antigenic viruses are produced that are immune to the antigens in the body of the host who was exposed to the earlier strains. Thus re-infection is common because of this property of antigenic drift produced by the endless changes in the antigenic and genetic properties of the virus. Due to this vaccine protection becomes difficult as they become ineffective on the newer mutations. Antigenic shift refers to the changes that occur in the surface antigens, the neuraminidase and hemaglutinin proteins that are seen in the different subtypes like the H1, H3, N,1 N2 (Palese & Young, 1982). These two proteins form the basis for the study of the epidemic or pandemic influenza virus, and in producing vaccines to counter them.

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