Preface -- 1. Revolutionary struggle for existence: introduction to four intriguing puzzles in virus research -- 2. Quasispecies dynamics of RNA viruses -- 3. The origin of virions and virocells: the escape hypothesis revisited -- 4. Scratching the Surface of Biology's Dark Matter -- 5. Virus universe: can it be constructed from a limited number of viral architectures -- 6. The addiction module as a social force -- 7. Viral integration and consequences on host gene expression -- 8. Persistent plant viruses:  molecular hitchhikers or epigenetic elements? -- 9. The concept of virus in the post-Megavirus era -- 10. Unpacking the baggage : Origin and Evolution of Giant Viruses -- 11. Megavirales composing a fourth domain of life: Mimiviridae and Marseilleviridae -- 12. On viruses, bats and men: a natural history of food-borne viral infections -- 13. LTR retroelement-derived protein-coding genes and vertebrate evolution -- 14. Koala retrovirus endogenisation in action -- 15. The evolutionary interplay between exogenous and endogenous sheep betaretroviruses -- 16. Endogenous retroviruses and the epigenome -- 17. From viruses to genes: syncytins -- 18. Hepatitis G Virus or GBV-C: A Natural Anti-HIV Interfering Virus -- 19. Salutary Contributions of Viruses to Medicine and Public Health -- 20. From Molecular Entities to Competent Agents: Viral Infection-Derived Consortia Act as Natural Genetic Engineers.

A renaissance of virus research is taking centre stage in biology. Empirical data from the last decade indicate the important roles of viruses, both in the evolution of all life and as symbionts or co-evolutionary partners of host organisms. There is increasing evidence that all cellular life is colonized by exogenous and/or endogenous viruses in a non-lytic but persistent lifestyle. Viruses and viral parts form the most numerous genetic matter on this planet. Viruses have long been considered as disease causing pathogens with often epidemic consequences and major enemies of living organisms. Viruses are now considered to play major roles in the evolution of life. Because they have genes that are not found in any cellular organism they seem to be remnants of early stages of life on earth. Besides their disease causing features and actions as genetic parasites viruses have lifestyles that are clearly symbiotic and even symbiogenetic. Increasing empirical data suggest that some viruses such as endogenous retroviruses and non-retroviral RNA viruses and even DNA viruses prefer cellular genomes as habitat. They determine genetic host (group) identity and genetic host features. Viruses and virus-related modules such as mobile genetic elements and other repeat sequences identified in intronic regions of host genomes play important roles in gene regulation and genetic content (re)arrangement. This book exemplifies some astonishing key features of viruses acting as essential agents of life.