Wired explains the science:
Bornaviruses, a type of RNA virus that causes disease in horses and sheep, can insert their genetic material into human DNA and first did so at least 40 million years ago, the study shows. The findings, published January 7 in Nature, provide the first evidence that RNA viruses other than retroviruses (such as HIV) can stably integrate genes into host DNA...
Retroviruses make up about 8 percent of the human genome. When these viruses insert into the genome, the result is usually bad for the host. But not always: Some retrovirus proteins can help fight off infection with other retroviruses. And at least twice in primate evolution retrovirus insertions have added genes to the host genome that aid in making the placenta. Now those proteins are essential for placenta development, says Cédric Feschotte, an evolutionary geneticist at the University of Texas at Arlington.
And as I did, I became less afraid of it. And I believe that lack of fear and sometimes over-dedication to my work helped me survive it, or, rather, survive with it. I think this is wiser than some modern conceptions of our bodies as somehow alien to illness rather than as part of the web of virology and bacteria and life that exists on this planet. So you tend to your illness as part of your body rather than seeing it as an invader to be conquered. I don't mean some new agey idea that modern medicine is irrelevant. Au contraire. HIV kills you if left untreated and aggressive medicine is vital. I mean a psychological resistance to the war metaphor, an ownership of your own cells, and the spiritual calm that accepts our physical embeddedness and is often the reward of a close call with mortality.
The calm fades so easily so often. But without it, we die.