It turns out that the baby gut microbiome is loaded with all sorts of viruses, and most of the species were unknown till now. All the viruses living in the gut is the virome. Analyzing baby poop is a way to find out what viruses live in the gut (intestines) of babies.
A team of scientists did an in-depth analysis of the poop (from diapers) of healthy one year old Danish children and found more than 10,000 new virus species! The overwhelming majority of the viruses are phages, which are viruses that attack and inhabit bacteria. There were many more viruses than bacteria in the baby poop.
Most of the phages are harmless, but some others are not so harmless, and this results in immune responses from the human host. In other words, all of this is normal and part of "training" the immune system in early childhood. The researchers named the newly discovered viruses after children participating in the study (e.g., Amandaviridae, Andyviridae).
By the way, it is normal for multitudes of viruses, bacteria, and fungi to live in the gut of humans throughout life - it's the gut microbiome.
Excerpts from Washington Post: Scientists identify thousands of unknown viruses in babies’ diapers
Research involving Danish babies’ dirty diapers has provided a plethora of information on previously unknown viruses — and the best view yet of the makeup of the infant gut microbiome.
The study looked at the feces of 647 healthy Danish 1-year-olds enrolled in a long-term asthma and chronic inflammatory disease study. The kids’ dirty diapers yielded a surprisingly diverse set of viruses — many of which have yet to be described by science. Overall, the researchers uncovered 10,000 viral species from 248 viral families; of those families, just 16 were already known.
The viruses were 10 times more abundant than the bacterial species in the children’s feces; 90 percent of them were bacteriophages, which attack bacteria instead of human cells. These bacteriophages don’t cause disease; instead, they are thought to shape bacteria’s competitive abilities and balance bacterial populations within the gut’s microbiome.
Why are so many viruses in kids’ guts to begin with?
“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,” Shiraz Shah, a senior researcher at the Copenhagen Prospective Studies on Asthma in Childhood and the study’s first author, said in a news release.