In July, scientists reported that a strange protein courses through the veins of pregnant women. No one is sure what it’s there for.

What makes this protein, called Hemo, so unusual is that it’s not made by the mother. Instead, it is made in her fetus and in the placenta, by a gene that originally came from a virus that infected our mammalian ancestors more than 100 million years ago.

Hemo is not the only protein with such an alien origin: Our DNA contains roughly 100,000 pieces of viral DNA. Altogether, they make up about 8 percent of the human genome. And scientists are only starting to figure out what this viral DNA is doing to us.

Aris Katzourakis, a virologist at the University of Oxford, and his colleagues recently published a commentary in the journal Trends in Microbiology in which they explored the possibility that viral genes that produce proteins like Hemo are affecting our health in a variety of unexpected ways.