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)

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

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

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

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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.

Written by: Dr Margaret Stearn
Edited by: Dr Margaret Stearn
Last updated: Thursday, March 4th 2010

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Comments on this article

Posted by Sigrid on 27/05/2010 at 03:18:06 pm

I seemed to have the outbreaks under control for a number of years now with almost being able to predict an outbreak however, just this year they cold sores seem to recur fairly quickly one after another. I know stress can be a huge trigger but I wonder if my age and coming into menopause might also contribute which I guess will not leave me with much prevention options?

Posted by Optional on 19/05/2010 at 10:30:20 am

how do you get rid of them completly or will they still come back?

Posted by Optional on 17/04/2010 at 06:53:34 am

Handy hint. If you do suffer with cold sores, avoid vitamin C! Everytime I go near a citrus fruit I get a massive coldsore! I told my friends who suffer and they've noticed too! We cant all be wrong or wired up the same way!

Posted by Optional on 26/03/2010 at 12:33:33 pm

A great tip that works for me when ive got them is to put vasaline on it at night, then by the morning has normally cleared.

Posted by Optional on 05/03/2010 at 03:50:31 am

Avoid dehydration and you'll avoid most outbreaks.... I get them EVERY time I go on a Big drinking binge (As I get severely dehydrated). I have also heard that Valtrex is a wonder pill.

Posted by becky on 10/12/2009 at 01:23:00 pm

Hi. I always seem to get cold sores in the corners of my mouth. As i am a naturally tired person, i like to yawn a lot. It really hurts when i yawn & it often starts bleeding...it's tearing me apart. Please HELP!!!

Posted by zoe on 14/11/2009 at 11:56:00 pm

my father is 40 and he has only just started getting cold sores is that normal?

Posted by Anonymous on 23/07/2009 at 05:27:00 pm

why do i get cold sores on my nose

Posted by Dean on 22/04/2009 at 04:21:00 pm

I have suffered from Coldsores for as long as I can remember...Here's my top tips for keeping them at bay; Sleep well - Being tired really increases the chances of getting a coldsore. Drink plenty of water. In my experience being dehydrated is 'the' number one cause of coldsores...I have reduced my coldsore outbreak from 6/year to 1/year (and that 1 is usually because I've forgotten to drink well enough). Coldsores have a high chance to breakout when it's sunny...i.e. You dehydrate faster. Coldsores have a high chance to breakout when you have drunk alcohol. Two-fold problem here, you get both tired and dehydrated from the alcohol. Drinking plenty of water after a night out, will help with your headache and coldsore problem the next morning. Good luck, fellow sufferers. ;)

Posted by emma on 21/02/2009 at 12:11:00 am

i have suffered with coldsores ever since my bf had one about 3 years ago and he hasnt had one since and i am still getting them is that normall?

Posted by Anonymous on 04/02/2009 at 05:21:00 pm

I have suffered from cold sores all my life... and quite severely! My father recently bought me a "Oral light" (also used for ulcers in the mouth). It is a instrument that looks much like a electric toothbrush but with a transparent end where the brush should be. Apply this end to where you feel the tingle for 1-2min every for often as you like. You feel a slight warm sensation. It works wonders for me!! Sometimes if you get it early enough you don't even blister! I use it in conjunction with creams. Wow. It's an amazing contraption. I love it and don't go any where without it. look at this link for what it looks like: http://209.85.229.132/search?q=cache:hm2qulpMx-0J:www.biolight.co.il/catalogs/oral.pdf+Oral+light+cold+sores+ulcers&hl=en&ct=clnk&cd=1&gl=uk&client=firefox-a This is not the one I used but looks similar.

Posted by amy on 04/02/2009 at 02:30:00 pm

i have a coldsoe n my cheek, on the left hand side. its sore and is starting to look yellow is this normal?

Posted by er on 03/02/2009 at 08:05:00 pm

i know i have some type of herpes, but i never went to the doctor. iI KNOW ITS BAD, BUT IT IS SO EMBARSSING....

Posted by SAVLON on 29/01/2009 at 08:59:00 am

Like many people I suffered the embarassment of large open sores.I have tried patches [ seemed to make matters worse by spreading to other parts of the mouth ] and various creams all seem to help but did not clear up the problem. My solution SAVLON I have found it rapidly clears and cleans the wound sometimes stopping it in its tracks and not developing any scab or sores, its purpose is to heal skin and I have found it works for me. I have not tried any similar products like germolene etc. so I can not comment on these products but as each persons skin will react differently its worth trying the various options. I can only restate my experience of almost immediate healing by using SAVLON. I started using it with a cream to heal the skin then using it instead of the creams.If you suffer badly then give it a try it might work for you. I hope this helps you it is not an endorsement of SAVLON merely an honest desire to share a solution that works for me. thanks AM

Posted by vicky p on 14/01/2009 at 11:36:00 am

can cold sores (herpes) be caught by having oral sex ?