Infl uenza A viruses are dynamic and can evolve by two processes, antigenic drift and antigenic shift. This capability of antigenic variation is responsible for the severe outbreaks of disease. Antigenic drift occurs by point mutations in the two genes coding for HA and NA and this causes minor changes in surface proteins. This leads to a new strain which is not recognized by antibodies to previous infl uenza strains. Antigenic shift is a major change through genetic reassortment which produces a novel infl uenza A subtype in humans. This occurs through mixing of human and animal infl uenza A virus genes or by animal to human transmission. A pandemic occurs when a new type of infl uenza A virus is introduced in humans that can cause a serious illness and is capable of sustained human to human transmission.