Science Questions

What is a cold sore?

Sun, 27th Jun 2010

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Caroline asked:

I have a question about cold sores. First of all, what is a cold sore and how come some people are most susceptible to them than us, than others?


Chris: -  The reason we get cold sores is because of a virus.  It’s a virus called Herpes Simplex Virus, HSV.  It’s one of the commonest human viruses.  In fact, about 80% of adults have antibodies to this virus, indicating they've been infected.  James Bond said, “Diamonds are forever.”  Well Herpes is for life.  Once you've got it, you have it for the rest of your life and the reason for that is that the virus exists as what’s called a latent state inside the cells of your nervous system.

Herpes simplex goes into the sensory nerves, the ones that supply the skin for example, and after you’re infected, the virus goes along inside the nerve fibre to the cell body which is a structure where the nucleus of the cell is, and inside that nucleus is the DNA of the cell, and the virus adds its DNA alongside your own cell’s DNA.  So it just sits there inside the cell and periodically, and in response to various stimuli, the virus can come back out again. 

So if you get sun burned, if you get trauma to the skin, if you get rundown, or ill for some other reason, this can all prompt the virus or provoke the virus to switch on the DNA.  In that DNA there are genes that tell it how to make new virus particles and they come back out down the nerve cell to the patch of skin that that nerve supplies, and the virus particles bud off from the end of the nerve, and infects the overlying skin, and you get a cold sore.  That’s got lots of virus particles, millions of virus particles in it, and they're infectious. 

You can then – if you get close to someone else, as in kissing close, you can pass the virus on.

Caroline: -  Oh right.  Okay.

Chris: -   So I guess you want to know why do some people get it, some don't?


Caroline: -   Yes.


Chris: -   We don't know  - because if you do this test, although you can find 80% of people have got it, only about 15% of people have regular so-called “reactivations.”  In other words, the virus only comes back periodically in about 15% of the people who’ve got it, and in an even bigger proportion, it can come back asymptomatically.  You can shed the virus without realising it, in saliva.  So it looks like there’s either something about the virus that makes it come back in some people - there might be some genes that are slightly different in the virus or more likely, is that there’s something different about the people that reactivate the virus much more frequently, and maybe they carry a gene or something that makes them more likely to reactivate it.  But the bottom line is that it’s normally getting rundown or sunburnt, or trauma to the skin that discloses the virus.


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