Viruses are endowed with a great ability to adapt to different environments. It means altering their cellular tropism and crossing host animal species barriers. The basis of their evolutionary success lies in subtle genetic changes through mutations and major genetic changes through recombination and nucleotide substitution. Genetic recombination is one of the primary processes that produce the genetic diversity upon which natural selection acts. RNA viruses have a great potential for genetic variation, rapid evolution and adaptation. Since influenza viruses have segmented genome, the reassortment is among important mechanisms for generation of new hybrid strains and/or subtypes. The modifications of viral RNA genome are frequently dependent on RNA polymerases erroneous action during replication, on the evolutionary pressure, on the novel environment of the host, the immune pressure, or antiviral drugs pressure. Shift to the new host species and virus transmission by insect vectors induced changes due to positive selection of variants with higher fitness for host-virus or vector-virus interactions. This article covers basic aspects of genetic diversity mechanisms of mutation and recombination of selected RNA viruses. The Influenza virus, Denga, Zika, West Nile, Rabies and SARS‑CoV-2 are just among several examples of virus genetic variations, that influence rapid evolution of viral diseases, and make design of reliable vaccine quite difficult