We know so little about the viruses in the human microbiome that a study just reported a newly discovered gut virus found in most of the world's population. From Medical Xpress:
Newly discovered gut virus lives in half the world's population
Odds are, there's a virus living inside your gut that has gone undetected by scientists for decades. A new study led by researchers at San Diego State University has found that more than half the world's population is host to a newly described virus, named crAssphage, which infects one of the most common types of gut bacteria, Bacteroidetes. This phylum of bacteria is thought to be connected with obesity, diabetes and other gut-related diseases.
The fact that it's so widespread indicates that it probably isn't a particularly young virus, either. "We've basically found it in every population we've looked at," Edwards said. "As far as we can tell, it's as old as humans are." He and his team named the virus crAssphage, after the cross-assembly software program used to discover it.
Some of the proteins in crAssphage's DNA are similar to those found in other well-described viruses. That allowed Edwards' team to determine that their novel virus is one known as a bacteriophage, which infects and replicates inside bacteria—and using innovative bioinformatic techniques, they predicted that this particular bacteriophage proliferates by infecting a common phylum of gut bacteria known as Bacteriodetes.
Further details about crAssphage have been difficult to come by. It's unknown how the virus is transmitted, but the fact that it was not found in very young infants' fecal samples suggests that it is not passed along maternally, but acquired during childhood.