Lurking between the genes of some of us are "non-human" pieces of DNA left by viruses that infected our ancestors hundreds of thousands of years ago.

And US scientists have reported how one stretch of DNA they found in about 50 of 2500 people they studied contained an intact, full genetic recipe for an entire virus.

While it's not yet known whether it can replicate or reproduce, other studies of ancient virus DNA have shown it can affect the humans who carry it. The scientists also confirmed 17 other pieces of virus DNA found in human genomes by other scientists in recent years.

The study, published in Proceedings of the National Academy of Sciences, looked at the entire span of DNA, or genome, from people from around the world, including Africa, where the ancestors of modern humans originated before migrating.