There is growing evidence that viruses have contributed to host evolution early in the evolution of life and throughout the subsequent evolution of biodiversity. It is important, from both a biological and medical perspective, that we grasp the essential principles involved. While virologists are familiar with the contribution of mutation to the evolutionary dynamics of the virus-host interaction, they may be less familiar with the role of partnerships and the principles and dynamics of symbiosis. We believe that an understanding of viral symbiosis complements the existing understanding of virology and offers the potential for novel virological research. This is of particular importance at a time when animal genomes, and the human genome in particular, have been found to contain large amounts of virus-derived DNA, formerly dismissed as junk but now increasingly recognized as contributing to host evolution, embryogenesis and physiology. In this paper we define the concept of viral symbiosis, clarifying the general principles involved before focusing on the particular example of the ERVWE1 locus in human evolution. From this we extrapolate the general principles to future biological and medical research.