Most species of viruses are now known to have common ancestors, and although the "virus first" hypothesis has yet to gain full acceptance, there is little doubt that the thousands of species of modern viruses have evolved from less numerous ancient ones.
Nüüd on teada paljudel viirustel on ühised eellased ja kuigi "viirus esmalt" hüpotees veel ei saanud tunnistust, peaaegu pole kahtlusi, et tuhat kaasaegset viirusi arenesid vähem arvukatest iidsetest.