The threat of a cold sore strikes fear into the heart of anyone who has had the misfortune to experience one. Not only are they disfiguring and painful, they are also prone to appear at the worst possible moment: just before an interview would be prime timing.

What causes cold sores?

The herpes simplex virus (HSV) - shingles is caused by herpes zoster - is responsible for this particular blight on human existence. The virus passes through the skin, travels up a nerve and hides in the nerve root until it’s activated. At the least excuse - such as tiredness or being out in the sunlight - it multiplies and travels down the nerve sheath to cause cold sore blisters, most commonly on or around the mouth.

How do you know if you’ve got a cold sore?

Once it appears there won’t be much doubt about what it is. There are four distinct stages to its life: tingling, blistering, weeping and scabbing.

The whole miserable process can take up to about 10 days.

How are they treated?

The condition can be hard to treat with convincing effect. Using a cream containing aciclovir, which can be bought over the counter from a pharmacy or on prescription, may reduce the length of the outbreak. However, it needs to be started as soon as the first symptoms appear. If you suffer really badly, you might also persuade your GP to part with anti-HSV agents in pill form.

How can they be avoided?

The virus can only be transmitted by close personal contact, such as kissing, so you are highly unlikely to pick it up from an afflicted pupil.

For many people, a childhood source is responsible for the virus entering their system, where it may have lurked for some time before presenting itself in the unwelcome guise of a cold sore. It is then important to avoid spreading the virus to other parts of the body by avoiding direct contact with the sore. If you do touch it - no picking allowed - wash your hand thoroughly afterwards.

In theory, the likelihood of repeat attacks can be minimised by recognising the factor that triggers the virus and avoiding whatever it is. However, since these often include such things as colds, stress and strong sunlight, praying to Saint Jude may prove as effective.

WASH THEM AWAY

* About 12 million people in the UK get cold sores - usually between one and three times a year.

* They are caught through close contact - usually kissing. You can’t catch them from cups, flannels or towels. After the first infection, the virus settles in a nearby nerve sheath and remains there for the rest of your life.

* Most cold sores will persist for a maximum of 10 days and rarely leave lasting scars.

* Recurrences tend to become less frequent with age. If the condition spreads to the eyes it can damage your vision, so it is important to wash your hands after contact with a sore.