In 1940s, Maurice Hilleman discovered antigenic drift, which is the most common way that influenza viruses change.[1][2][3][4] A second type of change is antigenic shift, also discovered by Hilleman,[1][2] where the virus acquires a completely new version of one of its surface-protein genes from a distantly related influenza virus. The rate of antigenic drift is dependent on two characteristics: the duration of the epidemic, and the strength of host immunity. A longer epidemic allows for selection pressure to continue over an extended period of time and stronger host immune responses increase selection pressure for development of novel antigens.[5]