Virus

Viruses undergo genetic change by several mechanisms. These include a process called genetic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are silent in that they do not change the protein that the gene encodes, but others can confer evolutionary advantages such as resistance to antiviral drugs.[65] Antigenic shift is where there is a major change in the genome of the virus. This occurs as a result of recombination or reassortment. I found this at Wikipendia