Novel strains of infl uenza virus arise due to antigenic shifts and drifts. These strains have very different surface glycoproteins which did not exist in human strains before. A pandemic occurs when such a virus emerges in humans with effi cient human to human transmission. As there is very little or no immunity against it, the virus quickly infects a large number of individuals in all age groups. A pandemic has been expected for long and it was feared that H5N1 avian infl uenza virus which caused severe disease in clusters of humans was the most likely candidate virus to cause a pandemic. For any pandemic to start three conditions need to be met: