There may be a virus dwelling in your gut, and you've probably never even heard of it. Then again, neither have most people.
While the virus seems to be extremely common, it was only discovered recently, when an international team of researchers studying bacteria stumbled upon it.
A paper describing the virus, called crAssphage, was published online Thursday in the journal Nature Communications.
The virus is a bacteriophage, or "phage," which means it infects bacteria. CrAssphage infects bacteria of the genus Bacteroides, which live in the human gut.
"It's in about 50 to 75 percent of the population," researcher Rob Edwards, a professor of bioinformatics at San Diego State University, says in a YouTube video describing the new discovery. Check out the video above.
Edwards and his team came across the virus -- named after a computational tool that first analyzed it -- while studying genetic samples collected as part of a National Institutes of Health project to investigate the various types of bacteria in the human body.
To gauge the virus' prevalence in humans, the researchers analyzed stool samples from the U.S., Europe and South Korea.
"As far as we can tell it's as old as humans are," Edwards says in the video. "It's very widespread. It's in every continent that we've looked at."
The phage is believed to be about 10 times the size of HIV, a relatively small virus. Its other characteristics are still a mystery -- like how it's transmitted. And it's unclear whether the virus is harmful to humans, or responsible for any health problems. But the researchers have their suspicions.
"We suspect this virus is very important in regulating the number of these bacteria [the Bacteroides] in the intestine," Edwards told NPR.
More research will be required to investigate what effect, if any, crAssphage has on the human body. In the meantime, the researchers are being praised for the sophisticated technique they employed to identify the virus.
"The biggest contribution of this work is the method they used," David Pride, associate director of microbiology at the University of California, San Diego (who was not involved in the study), told National Geographic. "It provides a blueprint for further viral discovery."