An ancient virus we don't yet fully understand has been discovered residing in the intestines of about half the world's population, scientists announced Thursday. Dubbed crAssphage, after the software that identified it, the researchers believe the virus may have something to do with obesity and diabetes.

A little more than half of all people in all regions of the globe have this virus, according to a press release. The just-released study appears in Nature Communications. “It’s not unusual to go looking for a novel virus" — as they did — "and find one,” co-author Robert A. Edwards of San Diego State University said. “But it’s very unusual to find one that so many people have in common. The fact that it’s flown under the radar for so long is very strange.”

Its far-flung distribution indicates crAssphage has been around for a long time — "as far as we can tell, it's as old as humans are," Edwards said. Newborns don't have it, which means our mothers probably aren't the ones passing it along. (We inherit much of our microbiome from our moms.) Likely those who are infected pick up the virus during childhood somehow.

It's interesting to think about what it might mean that a little more than half of all people have a particular virus that others don't. But any one microbe usually isn't very consequential. As far as scientists know, crAssphage doesn't directly cause illness. There's a chance, however, that it may play a role in whether a person will become obese or diabetic.

Viruses aren't alive; they have no cell structure. What they do have is DNA wrapped in protein. They replicate by hijacking the cells of living organisms, from plants to bacteria to animals. In the case of crAssphage, it seems to latch on to the cells of a class of lower-intestines-based bacteria called Bacteroidetes. At least one experiment on mice has shown that the number of Bacteroidetes in the gut decreases as mice become more obese. Others have demonstrated that the microbiome of the digestive tract changes rapidly when we eat.

What does crAssphage have to do with all that? Nobody knows. The presence of the virus could promote disease, suppress it or do nothing at all, and Edwards says future research will focus on that aspect. It's not crazy to think a cool new drug to combat weight-related disease might come from this. Here's Edwards' example: “In individuals, we could isolate your particular strain of the virus, manipulate it to target harmful bacteria, then give it back to you.”

Source: Edwards RA, Dutilh BE, Cassman N, et al. A highly abundant bacteriophage discovered in the unknown sequences of human faecal metagenomes. Nature Communications. 2014.