If the virus DNA remains intact, it still has the capacity to multiply. It may produce new viruses that break out of a cell, and even leap into a new host. But over the generations, the virus DNA may mutate and degrade. It may no longer be able to escape its own cell. But the virus may still have a bit of life left to it: it can make new viruses that insert their genes back into the genome at a new location.
This process has generated a huge amount of viral DNA in the human genome. We carry about 100,000 pieces of DNA that came from retroviruses–known as endogenous retroviruses. All told, they come to an estimated 5 to 8 percent of the entire human genome. That’s several times more DNA that makes up all 20,000 of our protein-coding genes.
There’s no way of telling if we are done with new endogenous retroviruses for good now, or if HIV or some other new retrovirus will manage to work its way into our genes. But the history of our inner viruses is still important to our health. Scientists have found HERV-K proteins made in tumors, suggesting that cancer cells may harness some of the biochemical power in these ancient parasites.