In the way we only hear about bacteria that's bad for us, we only hear about how viruses do nasty things to us. But it's well known that our relationship with viruses is not all bad. Indeed, in one particular instance, a protein from a virus plays a vital role in a process at the heart of human/mammal life: placental development and architecture.
Retroviruses have had a tremendous impact on animal genomes. At least eight percent of the human genome is comprised of retroviruses at various stages of “fossilization”. These elements represent retroviruses that have directly infected genomes of germline tissues such that their imprints can now be passed on with the rest of the genome. Most insertions into host genomes are likely to (i) be instantly so deleterious that they are never passed on, or alternatively (ii) have very little consequence to host biology and be expected to abrade away via the accumulation of mutations. Although the large fraction of retroviral imprints show expected signatures of mutational degeneration, some retroviral genes have been surprisingly preserved against mutational inactivation. These represent instances in which host genomes have usurped some retroviral genes for their own use. Particularly intriguing are host domestications of retroviral envelope (env) genes. The best-known classes of these genes are the syncytin genes, which have been coopted by the host to mediate nutrient transfer from the mother to the developing embryo in mammals...This not only means that we are embedded in a viral network (spatially and chronologically), but also that genetic innovation is not solely the consequence of random mutations in DNA replication—an idea that's old but still alive in some parts, as James A. Shapiro's Evolution: A View from the 21st Century makes clear. (I'm not done with this difficult book yet; more about it in the near future.)
For a light book on the current thinking about viruses, I recommend Carl Zimmer's A Planet of Viruses. For a heavy one, I recommend Luis P. Villarreal's Viruses and the evolution of life.
