The sexually transmitted disease genital herpes can be easily spread by people who carry the virus, but show no symptoms, scientists have discovered.

They warn that the spread of the disease - which has already reached epidemic proportions in the US - is likely to continue unabated unless everybody who carries the virus is aware they are infected.

Herpes, which is incurable, can cause painful blisters, ulcers or crusts in the genital area and the buttocks.

It is estimated that one in four Americans already has genital herpes, which is caused by herpes simplex virus type 2 (HSV-2).

Most genital herpes infections are acquired from a person with no history of genital herpes infection

Dr Anna Wald, University of Washington

However, most of them do not know they have been infected with the virus because they exhibit no symptoms.

It had been thought people who appear asymptomatic were less likely to infect others.

But new research from the University of Washington, published in the New England Journal of Medicine, has found that these people are just as infectious as those who have the symptoms of genital herpes.

Because the disease can lurk unseen or show only subtle symptoms, it is estimated that only one in five infected people knows they are carrying the virus.

Dr Anna Wald, director of the University of Washington's Virology Research Clinic, said: "Previously, many people thought that asymptomatic persons were less likely to shed HSV in the genital area than those with chronic symptoms.

"That is just not true. Even asymptomatic people shed a lot."

New strategy needed

Dr Wald said doctors tended to ignore people who did not show symptoms of genital herpes - and warned that this had to change.

She said: "In fact, most HSV-2 infections are acquired from a person with no history of genital herpes infection.

"In order to prevent the spread of HSV-2 to babies and to sexual partners we will have to identify and control the infection in people who do not currently realise they have the disease."

After the initial infection, genital herpes lurks without symptoms in nerves at the base of the spinal cord.

Only when it travels along the nerves to the skin, do symptoms appear.

Herpes can be suppressed with medication.

Although the disease is not fatal, it does allow easier transmission of the HIV virus.

It can also be passed to babies during pregnancy.

In the study, scientists found 53 people who had tested positive for HSV-2, but who were unaware that they had the infection.

More than four out of five cases were infectious.