This is an infection caused by the Herpes simplex virus (HSV), of which there are two main types. In general, HSV type I causes less severe genital symptoms than HSV Type II

Genital herpes is usually passed on through sex with an infected partner. It can also be transmitted through receiving oral sex from someone shedding Herpes virus from his or her mouth (they don't necessarily have an active lip cold sore).

Many people have the Herpes simplex virus realising - either because they have no symptoms or because they develop only mild soreness, which they dismiss as Candida (thrush). Antibody studies suggest that over 60 per cent of the population have been exposed to Herpes simplex infection.

Viral shedding from the genitals and mouth may be intermittent. The sudden appearance of genital Herpes in a relationship does not necessarily mean that a partner has been unfaithful. In fact, half of new cases of HSV occur in people in a stable relationship in which their partner was unaware they ever had herpes or that they were an asymptomatic carrier of the infection.

If someone knows they have genital herpes, and their partner hasn't had symptoms, the availability of blood tests to detect Herpes antibodies will help to show whether or not the non-infected partner is immune. This can help a couple decide whether or not to use condoms, for example.

What symptoms will I notice?
The first (primary) HSV attack is often accompanied by flu-like symptoms of aching, shivering and mild fever. You will often notice itching and tingling at the site of entry of the virus before a crop of painful blisters develops. These blisters slowly burst to form shallow ulcers, which are intensely painful as underlying nerve endings are exposed. Lymph nodes in the groin usually become enlarged and tender and eventually, the ulcers crust, scab over and heal after ten to fourteen days.

During the worst attacks, in which someone has a very low natural immunity to the virus, blisters may continue to appear for three weeks before healing starts, but this is unusual. Pain can make walking or sitting difficult, and often makes urination tricky - partly due to an effect on nerve function. It usually helps to pass urine while sitting in a warm bath, so that concentrated urine doesn't come into contact with raw sores. If urination is impossible, hospital admission is sometimes necessary for insertion of a catheter into the bladder until nerve function improves.

How are genital herpes diagnosed?
By swabbing a suspected viral blister and growing the virus in the laboratory. Treatment is usually started if herpes is suspected, as it takes several weeks before results are available. Culturing the virus is not easy and swabs often come back negative, especially if the sores are beginning to heal and no longer shedding virus.

New blood tests: These are available to detect HSV antibodies - they will show if you've been exposed to HSV in the past. A positive result does not necessarily mean that current symptoms are due to HSV, though.

How are genital herpes treated?
First attacks are usually treated with an oral antiviral drug such as acyclovir, famciclovir or valaciclovir, which help to reduce the duration of an attack and may help to suppress recurrences.

What if genital herpes recurs?
Once infected with HSV, around 50 per cent of people have no further problems. The remaining half develops one or more recurrences, in which the dormant virus becomes reactivated. The recurrence may occur weeks, months or even years after the initial infection. Some people only have one or two recurrences, while others get them regularly - occasionally as much as once a month or more.

Recurrent attacks are usually much less troublesome than the primary attack. Rather than several ulcers forming, only one or two small lesions tend to develop. These also heal more quickly - usually within five to seven days.

Warning signs: Known as a prodrome - of aching, tingling, gnawing or stabbing pains in the back or down the legs. Although recurrent ulcers tend to reappear in a similar site to the first attack, occasionally they appear elsewhere (for example, on another part of the labia, or even around the anus).

Triggers include:

• Stress
• Menstruation
• Ultraviolet light (don't use sun beds)
• Other infections (for example, viral illnesses such as the common cold)
• Vigorous sex

Treatment: Bathe the area with saline (salt water) in mild recurrences. More severe symptoms may require antiviral drugs - often applied as a cream rather than taken as oral tablets. If recurrences are frequent or severe, a course of an oral antiviral drug may be prescribed to help suppress recurrent attacks.

How does herpes affect pregnancy?
During pregnancy, if active genital herpes sores are present at delivery, there is a small chance (five to ten per cent) that the baby could acquire infection. This rarely causes widespread infection, such as viral meningitis or pneumonia. This is most likely to happen if it is a primary infection, however, as you won't have had time to develop antibodies that might help to protect your baby.

Tell your midwife/obstetrician if you've had genital herpes. They can advise on whether or not you might need a caesarean if active sores are present when labour starts. Some doctors don't recommend this as on the few occasions when a baby does become unwell, treatment with antiviral drugs is very effective.

If you're worried you have an STI or want to find out more, check out the symptoms, diagnosis and treatments for:

• Chlamydia
• Genital warts
• Gonorrhoea
• HIV and AIDS
• Pubic lice
• Scabies
• Syphilis
• Trichomonas