( Health.com ) -- People who carry the genital herpes virus but have no visible symptoms -- and may not even be aware they 're infected -- are still capable of spreading the virus about 10 % of the time , according to a new study in the Journal of the American Medical Association . An estimated 16 % of U.S. adults are infected with herpes simplex virus type 2 ( HSV-2 ) , which can cause bumps , blisters , or sores on the genitals and surrounding areas , along with pain and itching . But only about 10 % to 25 % of these people know they carry the virus . While some infected people have frequent outbreaks , most never have symptoms and others have just one or two flare-ups . Previous research has shown that people infected with HSV-2 can pass the virus to someone else through genital contact even if they do n't have symptoms . The new study -- the largest of its kind to date -- helps quantify that risk , says Christine Johnston , M.D. , one of the study 's authors and an acting assistant professor of medicine at the University of Washington , in Seattle . `` A lot of times people are diagnosed with HSV-2 by an antibody test and they do n't know what to do with that information , '' Johnston says . `` Now I think we have a number that we can tell people . '' Health.com : Top 10 myths about safe sex and sexual health . The study included 498 adults with healthy immune systems who had a diagnosis of genital herpes or who tested positive for HSV-2 . Every day for at least 30 days , the participants inspected themselves for symptoms and collected a swab of genital secretions , which were stored and later tested in a lab . The 410 people with a history of herpes symptoms were found to be actively `` shedding '' the virus on 20 % of the days for which they collected swabs , compared with 10 % of days for those who had never experienced symptoms . The symptom-free people released the same amount of virus when they were shedding as the symptomatic people did when they were n't experiencing an outbreak . ( The amount of virus increased if symptoms were present . ) The 10 % figure is an average , Johnston notes ; some people will shed the virus more frequently , while others will shed it less often . `` People will interpret the risk very differently , '' she adds . `` Some people will feel like 10 % is a lot and some people will feel like 10 % is a little . '' Health.com : 28 days to a healthier relationship . `` [ The study ] just begs the point that safe sex is important , and you had better know who your partner is , '' says Richard D. Whitley , M.D. , a past president of the Infectious Disease Society of America and a professor of medicine at the University of Alabama at Birmingham . Whitley is a leading expert on herpes but was not involved in the current study . Everyone should know whether they have HSV-2 , Whitley says . Aside from the risk of unwittingly infecting sexual partners , pregnant women with no symptoms can pass on the virus to their babies with devastating consequences for the newborn , including death . ( The risk of transmission is as high as 30 % to 50 % if the mother is infected during her third trimester . ) In addition , HSV-2 infection can cause complications in people who have weakened immune systems due to chronic disease , and it can increase a person 's likelihood of contracting HIV , Whitley says . Health.com : What to do if your partner refuses to wear a condom . People with HSV-2 can protect their partners from contracting the virus by using male or female condoms during sex , and by taking virus-suppressing drugs . Each measure reduces the risk of transmitting the virus to a sex partner by about half . Some people with herpes take antiviral drugs for years , while others just use them periodically -- for example , when they 're starting a new relationship or anticipating a high-pressure situation . ( Stress can up the risk of a herpes outbreak . ) `` It really is a very personal decision for patients , '' Johnston says . Copyright Health Magazine 2011 .
An estimated 16 % of U.S. adults are infected with herpes simplex virus type 2 ( HP ) Some people will shed the virus more frequently , while others will shed it less often . The infection can cause death in those with weakened immune systems .