A study into the gut microbiota of babies identified 10,000 new viral species on dirty diapers, with scientists saying these previously unknown viruses most likely play an important role in keeping the babies healthy.

For their study, scientists at the University of Copenhagen in Denmark spent five years examining the diapers of 647 healthy one-year-old babies enrolled in a long-term childhood asthma research project. They found and mapped a total of 10,000 viral species in the children’s diaper contents—a number 10 times larger than that of bacterial species in the same children.

These viral species are distributed across 248 different viral families, of which only 16 were already known to scientists. The remaining 232 newly discovered viral families are named after the babies participating in the study, including Amandaviridae, Hannahviridae, and Tristanviridae.

An overwhelming 90 percent of the viruses found in the study are what the scientists call “bacteriophages,” viruses that solely and selectively infect and kill bacteria. They do not attack the children’s own cells and therefore do not cause sickness.

According to the scientists, these bacteriophages very likely act as babies’ allies and play an important role in protecting them from chronic disease.

“Some bacteriophages can provide their host bacterium with properties that make it more competitive by integrating its own genome into the genome of the bacterium. When this occurs, a bacteriophage can then increase a bacterium’s ability to absorb various carbohydrates, thereby allowing the bacterium to metabolise more things,” explained Dennis Sandris Nielsen, the study’s co-author and a food science professor at the University of Copenhagen.

“It also seems like bacteriophages help keep the gut microbiome balanced by keeping individual bacterial populations in check, which ensures that there are not too many of a single bacterial species in the ecosystem. It’s a bit like lion and gazelle populations on the savannah,” the professor added.

The remaining 10 percent of viruses do infect human cells, apparently without making children sick. While their exact role remains unclear, it is possible that they serve to train the immune system to recognize infections. It is also possible that they are linked to unknown diseases.

“My guess is that they’re important for training our immune system to recognise infections later. But it may also be that they are a risk factor for diseases that we have yet to discover,” says Nielsen.

When it comes to why so many viruses are in babies’ guts in the first place, scientists said this probably has to do with the children’s undeveloped immune systems.

“Our hypothesis is that, because the immune system has not yet learned to separate the wheat from the chaff at the age of one, an extraordinarily high species richness of gut viruses emerges, and is likely needed to protect against chronic diseases like asthma and diabetes later on in life,” said Shiraz Shah, the study’s first author.

The study was published on April 10 in Nature Microbiology.