Cold sores are a very common condition, caused by a type of the herpes virus.
People can contract cold sores around their mouth but also on the genitals. The vast majority of people have this type of virus in their body, having picked it up during childhood (it is passed on by skin to skin contact with someone with the virus: usually by kissing). Cold sores usually begin with a tingling feeling on the lip, and then turn into a blister or sore. These usually clear themselves up after a few days, but treatments (best used early) are also available over the counter at the chemist.
The virus that causes cold sores usually lies dormant and harmless in the body. However sometimes it can come back to the surface in the form of one of these sores. It can be triggered if someone is feeling run down or under stress. The virus that causes cold sores can be passed on even when no sore is visible.
There is another type of herpes that particularly affects the genitals (see sexually-transmitted infections – genital herpes). It is possible, though unusual, for the two types to cross over during an attack if unprotected oral sex takes place.