A cold sore, which is commonly referred to as a fever blister, is caused by the herpes simplex virus. Cold sores are contagious, inflamed fluid-filled blisters that can be found near your mouth or on your lips. It’s important to understand that the herpes simplex virus which causes cold sores is highly contagious through oral contact and it cannot be cured. This virus is usually dormant and only presents itself from time to time.