Influenza virus is a single stranded RNA virus, bound by a nucleoprotein.
This nucleoprotein determines the type of the virus whether it is virus type A, B or C.
Thespherical surface of virus is a lipid bilayer containing the viral antigens. The viral antigens are "hemagglutinin and "neuraminidase". These two antigens determines the subtype of virus that is H1N1, H3N2 etc.(H stands for Hemagglutinin and N stands for Neuraminidase).
When an individual gets infection of Influenza virus, antibodies are formed against the neuraminidase and hemagglutinin antigens, which prevent the future infections with same subtype of influenza virus.
But recurrent infection occur through mutations of hemagglutinin and nuraminidase antigens that allow the virus to escape out of the host antibodies, this phenomenon is called antigenic drift.
also see antigenic shift in influenza virus
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