Teens today have fewer herpes antibodies than those a decade ago, making them more susceptible to a serious infection when they become sexually active.

Today’s teens are more susceptible to genital herpes than teens a decade ago—but not for the reasons one might expect. Researchers say it’s due to a decline in childhood exposure to herpes simplex virus (HSV) type 1, which causes cold sores and is a growing cause of genital herpes.

In the past, children were regularly exposed to the herpes virus through skin contact with an infected adult or by sharing lip balm or other personal items. This exposure can start the production of antibodies that can come in handy when teens become sexually active and may be exposed to herpes from a partner. But efforts to reduce HSV-1 exposure—by discouraging sharing cosmetics (along other tactics)—have translated to a 23 percent decrease in HSV-1 antibodies over the past ten years.

The decline in exposure can also be attributed in part to improved living conditions in industrialized nations, the researchers say.

These findings were reported in a new study published in The Journal of Infectious Diseasesby researchers at the Centers for Disease Control and Prevention (CDC). The researchers used data from the National Health and Nutrition Examination Survey (NHANES), comparing teenagers between 2005 and 2010 with teenagers between 1999 and 2000.

“Almost one in 10 adolescents who 10 years ago already would have acquired HSV-1 earlier in life now are vulnerable to getting a primary infection as they enter their sexually active years,” writes David Kimberlin, M.D., the chair of infectious diseases at the University of Alabama at Birmingham School of Medicine, in an accompanying editorial.

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