Influenza virus which have undergone antigenic shift have also gone on to cause the Asian Flu pandemic of 1957, the Hong Kong Flu pandemic of 1968, and the Swine Flu scare of 1976. It has been over 37 years since the last pandemic in 1968. One increasingly worrying situation is the possible antigenic shift between avian influenza and human influenza. This antigenic shift could cause the formation of a highly virulent virus.