A man or woman may not catch genital herpes despite years of having sex with an infected mate, according to a study that contradicts the widely held belief the infection is easily transmitted.

The study, however, also raised a danger signal. It said an uninfected woman with an infected husband can catch the virus at precisely the wrong moment: during pregnancy. If passed on to the baby during birth, the virus can cause death or severe illness.

Researchers found that among married couples, one spouse is frequently infected while the other is not, and the herpes-free mate may stay that way for years.