Thing about viruses in general is that they don't have DNA, only RNA. In more complex organisms, DNA codes for RNA codes for protein. The extra step in transcribing packaged DNA to code as RNA acts as an error check. Lacking that error check makes it very likely for a virus to mutate -- double a string, omit a string, invert a string, or stick a string in the wrong place and you have a mutation. Most mutations are either useless or lethal to the virus but a tiny fraction of them make it suited to do something new -- "novel" -- and the only way for us to combat this is to, through some population acquiring the sickness, develop antibodies. Vaccines come after that, by killing the virus with antibodies or other agents, to produce inactive virus that can stimulate the production of more antibodies in newly-protected would-be hosts. This is also why there's a new flu vaccine every year -- that virus is also continually mutating. If you've ever been inclined to disregard Evolution as a mere "theory", here is an example that shows it in action, all around you, all the time.