Psst! Don't panic, but you've got sleeper agents inside of you: viruses that have integrated their genome into human DNA. No big deal, you say, so what if we've got a few hitchhiking genes in our genome? Actually, there's quite a bit of them:
Scientists have identified 100,000 pieces of retrovirus DNA in our genes, making up eight percent of the human genome. That’s a huge portion of our DNA when you consider that protein coding genes make up just over one percent of the genome.
Carl Zimmer of The Loom blog over at National Geographic explains how one such viral gene could be awaken (by infection by HIV, for example) after lying dormant for six million years in our DNA:
Read the rest over at The Loom: LinkMarkovitz and his colleagues analyzed the sequence of the virus genes they found in the patients with HIV. The genes belonged to a family of endogenous retroviruses called HERV-K, but they were not quite like any known HERV-K virus previously found.
The Michigan scientists wondered if this new HERV-K virus was hidden in the human genome. They checked the most complete draft of the human genome and couldn’t find a match. They knew that the human genome sequence was only about 95% finished, so they turned instead to the chimpanzee genome, on the off chance that the virus had infected the common ancestor of humans and chimpanzees over six million years ago. Bingo: a single copy of the virus turned up in the chimp genome. They dubbed it K111.
Having found this match, the scientists decided to return to the human genome and search for K111. They isolated DNA from their HIV patients, as well as from healthy people. They then split apart the two strands of the DNA and added a short piece of DNA that would bind to K111, should it be lurking there. In all 189 of their subjects, the scientists found the virus’s DNA.