Most children get infected with human herpesvirus 6 (HHV-6) before their second birthday, and most of us still carry it as adults. In people with healthy immune systems, the virus does no lasting damage, though it can cause roseola—an illness characterized by high fever and a red rash. HHV-6 was unknown until 1986, and we still don't know much about it—most notably how it behaves in children. Researchers from Seattle examined infected children to try to see if they could shed light on this common but little-known virus.

What the researchers wanted to know: How do children contract HHV-6, and what does it do to them?

What they did: The researchers tracked 277 children from birth to age 2. Parents collected the child's saliva once a week and mailed it to the researchers, so the researchers could tell when the kids had been infected. They also filled out daily symptom reports about whether their children had a fever, a cough, diarrhea, a rash, or any of various other symptoms. (The researchers used a record of fever and rash to identify roseola.) The researchers also asked parents whether the babies had siblings, if they were breast-fed, and if they attended child care or played with other children.

What they found: By age 2, 77 percent of the children were infected with HHV-6. Most of the infections occurred between the ages of 9 and 21 months, and babies were more likely to get the virus if they had an older sibling. Nearly all of the babies became ill when they first got HHV-6—most had fever, fussiness, and a runny nose. About a quarter of the children developed roseola, and about half of those with HHV-6 visited a doctor. None of the children had serious symptoms at the time they were infected with the virus.

What it means to you: Yep, your child will probably get HHV-6 (if he doesn't have it already). Most children skate through a few days of illness none the worse for the wear, with only a fever, diarrhea, or possible rash. Some studies have linked this virus to more serious diseases—notably multiple sclerosis—but evidence is still inconclusive. And since 77 percent of us do not have multiple sclerosis, clearly it can't be the only factor.

Caveats: For this study to work, the parents had to be involved in a major way—collecting saliva and reporting daily on their child's health. Most parents didn't do it; on average, parents stayed in the study for just longer than one year. In addition, the researchers taught the parents how to collect their babies' saliva without contaminating it, but there's no way to tell if they followed those practices.

Find out more: A description of the virus, though quite technical, is available through the Centers for Disease Control and Prevention.

Kidshealth.org offers more practical—and less technical—advice, including several pages on roseola, which is caused by HHV-6

Read the article: Zerr, D. M. et al. "A Population-Based Study of Primary Human Herpesvirus 6 Infection." New England Journal of Medicine. Feb. 24, 2005, Vol. 352, No. 8, pp. 768–775

Abstract online: http://content.nejm.org