text: Mutations is viral strains are highly exhibited when they are retroviruses such as HIV or the H5N1 strain. Such viruses do not have DNA initially, much simpler for them. An enzyme called Reverse Transcriptase transcribes DNA from RNA when these viruses invade host cells. However, these is a 'stupid' enzyme and it makes mistakes, and for every mistakes it makes, another strain is created due to mutation. For the influenza virus, this is such a trivial phenomenon. It happens almost all the time. But since viruses are relatively much less complicated than eukaryotes, an error usually does not prove fatal. For mammals, there is a smart enzyme called DNA polymerase which transcribes RNA from DNA and the error checking mechanisms are so intricate. \nThe other way can be the kind sort of mentioned by one of the other answerer's. This gene transfer is the so called Horizontal Gene Transfer and it can result in a modified viral strain but I think that is the case mainly for DNA viruses.