Cold sores are caused by a virus called the herpes simplex virus (HSV). This virus is very common and it’s estimated to affect approximately 67% of the world’s population. The virus is caught by coming into contact with other people with an active infection. This is often during childhood when children come into close contact through activities such as play.
For most of your life, the virus will lie dormant in your skin, suppressed by your immune system. Occasionally the virus will reactivate causing a cold sore. This is often when your immune system is temporarily weakened, for example through stress or illness.