Cold sores are caused by the herpes simplex virus 1 (HSV-1). They occur around the mouth and lip area as small fluid filled blisters, and are transmitted from one person to another through physical contact. It is unlikely that you will know you have been affected with the virus straightaway, as once infected the virus will remain inactive. For some this can mean experiencing an outbreak anytime in the future, while for others this could mean never experiencing an outbreak at all. Cold sores can be treated with antiviral medication, which will help to relieve symptoms and reduce the time of the infection.