“Retroviruses, a large class of viruses that includes the human immunodeficiency virus (HIV), do the same thing in humans, splicing their genetic material into the genome of infected cells. This pernicious property makes retroviruses especially challenging to eradicate, so much so that they have left an outsize mark on our species. A full 8 percent of the human genome—over 250 million letters of DNA—is a remnant of ancient retroviruses that infected ancestors of our species millennia ago.”