These days, people are coming to grips with the fact that they live with untold numbers of bacteria both on and inside their bodies. A recent report even shows that each individual’s bacterial entourage moves in and takes residence in a short while wherever their human hosts decide to shack up. But are viruses also part of the scene?

New research sponsored by the National Institutes of Health (NIH) at the Washington University School of Medicine in St. Louis, Missouri, says yes, viruses are part of the human microbiome, or collection of microorganisms living in or on the human body. The findings represent some of the most recent accomplishments of the Human Microbiome Project, a major NIH initiative to sequence the genomes of the body’s bacterial residents.

“Most everyone is familiar with the idea that a normal bacterial flora exists in the body,” said study co-author Gregory Storch, a virologist and chief of the Division of Pediatric Infectious Diseases. “Lots of people have asked whether there is a viral counterpart, and we haven’t had a clear answer. But now we know there is a normal viral flora, and it’s rich and complex.”

In a study reported in the open-access journal BioMed Central Biology, Kristine Wylie and colleagues sampled 102 healthy young adults ages 18 to 40. They took from their volunteers nose, skin, mouth, stool, and vaginal specimens from both men and women in roughly equal proportions (the vaginal samples coming only from women, obviously). The researchers then assayed for viruses in the collected samples.

At least one virus was common to 92 percent of the volunteers. Some individuals harbored up to 10 to 15 viruses at once.

“We were impressed by the number of viruses we found,” said Wylie, instructor of pediatrics. “We only sampled up to five body sites in each person and would expect to see many more viruses if we had sampled the entire body.”