Most viral infections are as fleeting as a cold, but two things made the ancient ones unusual: One, these viruses had the special ability to copy themselves into the DNA of their hosts. And two, they sometimes got lucky enough to copy themselves into an egg that became fertilized and grew into a full-fledged adult. So that viral DNA got passed down from generation to human generation as so-called endogenous retroviruses.

But there's no need for alarm about the DNA of viral origin teeming inside your cells. Some of it may even make you you. As a growing fetus, you co-opted a gene from an ancient virus to form the placenta that kept you nourished in the womb. And in recent years, scientists poring over gigabytes of genetic sequencing data have seen other tantalizing hints of endogenous retroviruses turning useful.