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.