Cold sores are due to infection with the herpes simplex virus (HSV).
Facial herpes is spread by close physical contact. Most people will have come into contact with the virus between the ages of three and five (for example, through being kissed or hugged by a relative who has the virus), but only one in three of these will have a first episode with symptoms.
The HSV invades the outer layer of the skin (the epidermis), resulting in fluid-filled blisters to appear. From here, it travels along the nerve paths to the trigeminal ganglion, a bundle of nerves close to the inner ear, where it lies dormant.
When a person is infected with herpes for the first time, the episode is called a primary infection. Symptoms vary from none at all through to moderate discomfort.This first outbreak starts one to three weeks after the virus has invaded the skin and subsides within a few weeks.
If sores develop inside the mouth, as well as outside, it is often called gingivostomatitis.This should be treated with antiviral medicine and pain relief as sores and blisters lining the mouth and throat make it harder to eat and drink and take longer to heal, lasting up to 14 days.
The virus remains hidden in the nerves for the rest of the person's life and becomes active again from time to time. Triggers can include a fever (eg, a common cold), UV radiation (exposure to sunlight), extreme tiredness or lowered immune function. Some people have regular outbreaks or recurrences, while others have none. With time, these become less frequent.
The virus can spread until the sores are completely covered by scabs and the infection will usually be external.
When someone has an episode of herpes, either facial or genital, they should consider themselves infectious from the start of the episode to the healing of the last ulcer. During this time the virus can be transmitted to other people and, in rare cases, can be transferred to other areas of the body.
To help prevent spread, you should avoid:
For more people with just one or two blisters on their lip or near their nose, keeping it clean, dry and leaving it alone are all that is needed.
Facial herpes The New Zealand Herpes FoundationCold sores Medline Plus (USA)Cold sores NHS Choices (UK)