While some viruses can make us ill, we have viruses on and in our body at all times even if we're in good health, researchers have found.

Scientists at Washington University in St. Louis, conducting what they say is a first extensive analysis of the kinds and numbers of viruses found in healthy individuals, report that the average healthy individual is carrying at least five types of viruses.

The study undertaken within a project of the National Institutes of Health is known as the Human Microbiome Project.

While the project has previously been addressing bacteria in the human body, researchers said they wanted to do something similar with a focus on viruses.

"Most everyone is familiar with the idea that a normal bacterial flora exists in the body," says Gregory Storch, a study co-author. "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," he says.

In the study, 102 volunteers aged between 18 and 40, evenly split by gender, were first screened to assess their health and confirm they were free of any symptoms of acute infection.

Then samples were secured from five possible viral habitats of the body: the mouth, nose, skin, stool and vagina.

In 92 percent of the participants at least one virus showed up in the samples, while some had as many as 10 or 15 different viruses present.

"We were impressed by the number of viruses we found," says Kristine M. Wylie, the study's lead author. The study only delved into five areas of the body in each person. The researches expect to find more viruses if the study included the entirety of the body.

Sequencing DNA in the recovered viruses, researchers found that every individual presented a unique viral "fingerprint."

The permanent presence of viruses in and on the body could have either negative or positive effects, the researchers point out.

In a positive sense they may be keeping the body's immune system ready and primed to deal with any serious infections. On the other hand, the risk of disease may be increased by viruses that linger on the body for an extended period of time.

The study proves that knowing which viruses exist is crucial in determining how to keep a person healthy and prevent the occurrence of dreaded illnesses that call for medical attention. 

More research will be necessary but the study has provided an initial representation of the viruses that are present in healthy people naturally and normally, Storch says.