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Cold sores are blisters that appear on your lips and around your mouth. They are caused by a virus called the herpes simplex virus.

If you get cold sores, you were probably first infected with the virus when you were a child.¹ You may hear cold sores called herpes labialis or " fever blisters".

There are two types of herpes simplex virus: type 1 and type 2. Type 1 mostly causes cold sores.² Type 2 and type 1 can cause sores or warts on the genitals. Here we are looking at what happens if you get type 1 virus on or around your mouth.

You can catch this virus from other people through their saliva. When you first get infected, you may or may not get symptoms.³ But children may have quite a serious illness when they get infected with the virus. They may get some or all of these symptoms:

• Ulcers on their tongue, their lips and inside their mouth
• Pain when swallowing
• Fever
• Swollen glands in their neck
• Bad breath
• Loss of appetite
• Low energy
• Feeling irritable.

If your child seems ill with these symptoms, you should take him or her to the doctor. He or she may need treatment.

Once you're infected, the virus stays in the nerves in your face. It can cause new cold sores at any time. There are certain things that seem to trigger new cold sores:

• Throat infections
• Tiredness
• Stress
• Your periods, if you are a woman
• Sunlight
• Drugs that affect your immune system (such as steroids). Your immune system is the part of your body that protects you against infection.

If you get cold sores, you can pass the virus onto other people through your saliva.⁴ You're most likely to pass on the virus when you have cold sores. To reduce the risk of passing on the virus, you should avoid kissing when you have cold sores and be careful not to share cups, cutlery or towels. Also, avoid oral sex when you have a cold sore, because the virus can sometimes cause blisters on the genitals.³

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.