Review Article

Abstract. Influenza means ‘flu’, caused by RNA viruses of Orthomyxoviridae family which is an infectious agent of birds and mammals. It causes mild to severe symptoms including chills, fever, sore throat, muscle pains, headache, coughing, fatigue but about 33 % of the cases with influenza are asymptomatic. Occasionally it leads to pneumonia in both healthy and immunosuppressive person. Influenza is transmitted through the air by coughing, sneezing or creating aerosols containing influenza. It can also be transmitted by direct contact with bird droppings or nasal secretions or through contact with contaminated surfaces. Influenza now spreads all over the world and it is also known as seasonal epidemics. Several reports depicted both emergence and pandemic potential of the virus in the perspective of earlier pandemic influenza viruses of 1918 (H1N1), 1957 (H2N2) and 1968 (H3N2) by comparison of the available genetic sequence data. An avian strain named H5N1 raised the concern of a new pandemic after it emerged in Asia in 1990s. After several years ‘swine flu’, also known as influenza A / H1N1, emerged in Mexico, USA and several other nations. The principal objective of the present work is to investigate the evolutionary history of the viruses circulating in Asia and to understand the relationship between epidemiologic and evolutionary process within the affected human population