A cold sore or fever blister is an inflammation that forms on or around the lips and can last for more than a week. This extremely common virus is often contracted in childhood and once you get the virus it never completely goes away. It lies dormant in your body waiting for the right time to strike. There are multiple "triggers" that may cause a cold sore. People often report a "tingling" sensation around the site before the cold sore develops. Without treatment, the sore may last up to 2 weeks and can be extremely painful and unsightly.