Human herpesvirus 6, pictured above, is just one of numerous viruses found living in and on the bodies of healthy humans. The virus commonly causes illness in young children but is found in the mouths of some healthy young adults, where its presence indicates an active viral infection despite a lack of symptoms.
On average, healthy individuals carry about five types of viruses on their bodies, the researchers report online in BioMed Central Biology. The study is the first comprehensive analysis to describe the diversity of viruses in healthy people.
The research was conducted as part of the Human Microbiome Project, a major initiative funded by the National Institutes of Health (NIH) that largely has focused on cataloging the body's bacterial ecosystems. "Most everyone is familiar with the idea that a normal bacterial flora exists in the body," said study co-author Gregory Storch, MD, 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 102 healthy young adults ages 18 to 40, the researchers sampled up to five body habitats: nose, skin, mouth, stool and vagina. The study's subjects were nearly evenly split by gender. At least one virus was detected in 92 percent of the people sampled, and some individuals harbored 10 to 15 viruses.
"We were impressed by the number of viruses we found," said lead author Kristine M. Wylie, PhD, an 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."
Scientists led by George Weinstock, PhD, at Washington University's Genome Institute, sequenced the DNA of the viruses recovered from the body, finding that each individual had a distinct viral fingerprint. (Weinstock is now at The Jackson Laboratory in Connecticut.) About half of people were sampled at two or three points in time, and the researchers noted that some of the viruses established stable, low-level infections.
The researchers don't know yet whether the viruses have a positive or negative effect on overall health but speculate that in some cases, they may keep the immune system primed to respond to dangerous pathogens while in others, lingering viruses increase the risk of disease.