Influenza is an acute viral disease of the respiratory tract caused by viruses A, B or C which antigenetically distinct and there is no cross immunity between them. While humans may be affected by all the 3 viruses, in lower animals and in birds influenza A virus is of primary concern. Influenza A viruses are divided into subtypes based on 2 glycoproteins(Haem Agglutinin and Neuraminadase antigens) present on the surface of the virus.There are 15 Haemagglutinin(H) antigens (1-15) and 9 Neuraminadase (N) antigens(1-9). While all 15 H subtypes readily infect the birds, viruses having any of H1, H2 or H3 and any of N1 or N2 only affect the human usually. In fact, H1N1,H2N2 and H3N2 subtypes of influenza A virus have been associated with most of the widespread epidemics and all the epidemics.
Antigenic drift: Frequent point mutations of the genes encoding 2 surface proteins of influenza A viruses result in emergence of variants which escape existing immunity to previously circulating influenza viruses in the population. This is responsible for the frequent regional outbreaks of influenza and necessitates annual reformulation of influenza vaccines.Antigenic shift : Subtypes from different species can swap or reassert genetic materials and merge resulting in a novel subtype different from both parent viruses. If the novel subtype has sufficient genes from human influenza viruses which make it readily transmissible from person to person, it may cause highly lethal pandemics. There is evidence that the human influenza viruses for the last 3 pandemics contained gene segments closely related to those of avian influenza viruses.
Pandemics:1889(H1N1). 1918 (H1N1): Estimated death 50 million deaths worldwide (7million in India).1957(H2N2): Estimated death 1-4 million worldwide. 1968(H3N2): Estimated death 1-4 million worldwide.