In a medical sense, you’d be wise to steer clear of filoviruses, a group that includes the deadly Ebola, and bornaviruses, which cause neurological diseases. But in a genetic sense, it may not be possible to avoid them. A new study in PLoS pathogens shows that bits and pieces of these viruses have been floating around in the human genome, as well as those of other mammals and vertebrates, for millions of years.

It’s not that having genetic material left behind by viruses is odd—previous research had shown that viruses account for 8 percent of the human genome. But scientists thought most of that material came from retroviruses, which use their host’s DNA to replicate and leave some of their genetic material behind. What’s weird about this is that filoviruses and bornaviruses are not retroviruses—they’re RNA viruses, which don’t use the host to reproduce in the same way.