Antigenic shift is seen only in Influenza 'A' virus. Antigenic drift is seen in Influenza A and B. Antigenic drift involves "point mutation" in the gene owing to selection pressure by immunity in the host population. Influenza shows cyclic trend due to these antigenic variations Type A epidemics every 2-3 years Type B epidemics every 4-7 years Type A pandemics every 10-15 years