What is it?
Cold sores are small infectious blisters that develop on your lips or around your mouth. Between 56-85% of people carry the virus which causes cold sores by the time they are an adult.  The first infection usually happens as a young baby or in childhood, though there may not be any symptoms. In 20-40% of young adults who have the virus, the cold sore can come back between two and six times a year. 
What are the main causes?
Cold sores are caused by the herpes simplex or ‘cold sore’ virus. They are extremely infectious and you can easily pass them from person to person by close direct contact (for example by kissing). You are most likely to pass on the virus when the cold sore bursts, but they are still infectious until they have completely healed and disappeared.
After you have caught the virus, it doesn’t become active until it is triggered by something, and then one or more cold sores develop. Triggers are different for different people. Common triggers are stress, tiredness, an injury to the affected area, strong sunlight, and in women, monthly periods.
Some people have cold sores that come back a few times a year, while others have one cold sore and never have another. Some people have the virus in their body, but it is not triggered and so they never have symptoms. 
What are the symptoms?
Once the virus has been triggered, you may feel a tingling, itching or burning sensation around your mouth. Small fluid-filled sores will then appear, most commonly on the edges of your lower lip. Cold sores are usually quite mild, but can cause complications if drinking fluids becomes painful, as you can become dehydrated. The virus can also spread to other parts of your body and cause painful sores on your fingers called whitlows.
In young children, the symptoms may be a little different, with ulcers on the tongue, lips, gums and other places in the mouth. They may also have pain, be unable to swallow, drool and become dehydrated, as well as have a fever, general tiredness, loss of appetite and irritability. This is known as gingivostomatitis.  If you suspect your child has this condition, you should seek medical advice
What can I do about it?
You can leave a cold sore to clear up by itself, which it should do within 7-10 days. To help ease your symptoms and speed up the healing time, however, you can buy treatment creams from your local pharmacy. It’s best to apply the cream as soon as you feel the tingling, itching or burning sensation around your mouth.
Make sure you avoid close contact with people more likely to pick up infections, for example:
- newborn babies (never kiss a newborn baby if you have a cold sore)
- people with immune deficiencies, such as HIV.
- people receiving treatments that are known to weaken the immune system, such as chemotherapy. 
- Drink plenty of fluids to avoid dehydration.
- Avoid acidic or salty foods and eat cool, soft foods.
- If brushing your teeth is painfull, use antispetic mouthwash instead of a tooth brush and toothpaste.
- Dab cream onto cold sores rather than rubbing it in.
- Wash your hands using liquid soap and clean water before and after applying a cold sore cream.
- Avoiding touching your cold sores other than to apply cream, and don't share your cold sore cream with others.
- Don't share medication or items such as lipstick that come into contact with the affected area.
- You don't need to stay away from work or miss school, but avoid your cold sore coming into direct contact with other people.
- Stay away from people who are more likely to pick up infections.