Endogenous retroviruses are retroviruses thought to be derived from ancient infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome. Retroviruses are viruses that reverse-transcribe their RNA into DNA for integration into the host's genome. Most retroviruses (such as HIV-1) infect somatic cells, but some can also infect germline cells (cells that make eggs and sperm) and once they have done so and have been transmitted to the next generation, they are termed endogenous. Endogenous retroviruses can persist in the genome of their host for long periods. However, they are generally only infectious for a short time after integration as they acquire 'knockout' mutations during host DNA replication. They can also be partially excised from the genome by a process known as recombinational deletion.
Endosymbiotic ERV's in mammals
During pregnancy in vivipary mammals (all mammals except Monotremes), ERVs are activated and produced in high quantities during the implantation of the embryo. They are currently known to act as immunodepressors, protecting the embryo from its mothers immune system. Also viral fusion proteins apparently cause the formation of the placental syncytium in order to limit the exchange of migratory cells between the developing embryo and the body of the mother (something an epithelium will not do sufficiently, as certain blood cells are specialized to be able to insert themselves between adjacent epithelial cells). The ERV is a similar type of virus as HIV (which causes AIDS in humans), the immunodepressive action was the initial normal behavior of the virus, similar to HIV, the fusion proteins was a way to spread the infection to other cells by simply merging them with the infected one (HIV does this too). It is believed that the ancestors of modern vivipary mammals evolved after an accidental infection to an ancestor to this virus, that permitted to the fetus to survive the immune system of the mother. 
The human genome project found several thousand ERVs comprise by 24 families.
Human endogenous retroviruses
Human endogenous retroviruses (HERVs) are suspected of involvement in some autoimmune diseases, in particular with multiple sclerosis. Investigations also suggest possible HERV involvement in the HELLP syndrome and pre-eclampsia. There are many thousands of endogenous retroviruses within human DNA (HERVs comprise 0.03% of the human genome, with 98,000 elements and fragments). All appear to be defective, containing nonsense mutations or major deletions, and cannot produce infectious virus particles. This is because most are just long-lasting traces of the original virus, having first integrated many millions of years ago. However, there is one family of viruses that have been active since the divergence of humans and chimpanzees. This family, termed HERV-K(HML2), makes up less than 1% of HERV elements but is one of the most studied. There are indications it has even been active in the past few hundred thousand years, as some human individuals carry more copies of the virus family than others. But the absence of known infectious members of the HERV-K(HML2) family, and the lack of elements with a full coding potential within the published human genome sequence, suggests that the family is less likely to be active at present.
-  The Viruses That Make Us: A Role For Endogenous Retrovirus In The Evolution Of Placental Species (by Luis P. Villarreal)
-  Persisting Viruses Could Play Role in Driving Host Evolution
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- HERVd - human endogenous retrovirus database
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- Endogenous+Retroviruses at the US National Library of Medicine Medical Subject Headings (MeSH)