mutation, i.e., sudden change in molecular structure of RNA/DNA in microorganisms, especially viruses, which produces new strains; hosts previously exposed to other strains have little or no acquired immunity to the new strain; antigenic shift is believed to be the explanation for the occurrence of new strains of influenza virus, which occur by recombination or genetic reassortment of two different viral strains in a given host, and is associated with large-scale epidemics.