Cold sores

What causes cold sores?
Cold sores are caused by a virus called herpes simplex. (Herpes simplex viruses also cause genital herpes.) Most adults worldwide have been infected with this virus, usually when we were children and were kissed by someone with a tiny cold sore that they were not aware of. When a child first gets the infection, there may be no symptoms at all, or there may be a fever and a sore mouth that clears up after 2–3 weeks.

After the first infection, the virus often remains. We seem to have difficulty in getting rid of it completely. It travels away from the mouth using the nerves of the face as a pathway. When it reaches the clusters of cells at the end of the nerves (which are called ganglia), it stops. The herpes virus DNA then remains quietly in the ganglia. Fortunately, it does not damage the nerves or interfere with their function.

In some people (about 1 in 12), from time to time, the herpes virus travels back the way it came, along the nerve towards the mouth. This is called activation. But instead of going to the inside of the mouth, it takes a slightly different path and ends up in the skin of the lips or nearby. When it arrives in the skin, it causes a cold sore.

After the cold sore heals, the virus goes back up the nerve to the ganglion again, where it rests. But it can become activated again at any time, and travel down the nerves to the skin to cause another sore. The virus always travels up and down the same or nearby nerve pathways, which explains why cold sores always recur in roughly the same place.

Infection with herpes simplex virus causes sore mouth (or no symptoms)

Herpes virus travels up nerves to the nerve ganglion where it remains

At a later date, herpes virus may travel down the nerves to the skin, causing a cold sore

The cold sore heals, and herpes virus retreats to the ganglion again

At a later date, herpes virus may travel down to the skin again, causing another cold sore (recurrence)

What activates the virus to cause a cold sore? No one really understands why the virus in the ganglion suddenly wakes up from time to time, and decides to go to the skin and cause a cold sore. But, in some people, there are some definite triggers that make this happen, such as:

• bright sunshine
• wind
• other damage to the skin (such as ‘facial resurfacing’ procedures)
• emotional stress
• physical stress, such as having another illness
• major dentistry, such as having a tooth removed
• menstruation.