A cold sore is a small, painful raised area of small, fluid-filled blisters, usually where the lip joins the surrounding skin. The blisters are painful and dry up to make a yellow crust which gradually heals in a few days. Cold sores are usually found on the lips but you can get them in other places, such as in the nostrils, on the nose or around the outside of the mouth. Cold sores tend to last 5 to 7 days and can keep coming back. Most people only have them once or twice a year.
Cold sores are caused by a virus (a 'Herpes Simplex' virus). You first get this virus in childhood or early adulthood, and it may cause a fever and mouth ulcers (called 'primary herpetic gingivestomatitis'). About a third of people who get this 'primary' infection go on to develop cold sores in later life. However, many people with cold sores did not get this primary infection.
If you have a cold sore and are due to visit your dentist or hygienist, check with them first. Some dentists prefer not to treat patients with active cold sores as the affected area may be painful, and may crack and bleed during dental treatment.