What Causes Cold Sores on Lips?

Cold sores are caused by the herpes simplex virus. The first cold sores usually occur in childhood. The virus infects through the moist 'inner' skin that lines the mouth. It is commonly passed on by skin contact such as kisses from a family member who has a cold sore. After the first cold sore infection, the herpes simplex virus settles in a nearby nerve sheath and remains there for the rest of your life. For most of the time, the virus lies dormant (inactive) and causes no symptoms.

However, in some people the virus becomes 'active' from time to time. When activated, the virus multiplies and travels down the nerve sheath to cause cold sore blisters around the mouth. Some people have cold sores often, others only now and then. It is not clear what causes the dormant herpes virus to become active.