Cold sores are small, fluid-filled blisters that develop on the lips or around the mouth. They are unsightly but usually harmless, healing on their own within a couple of weeks. Caused by the herpes simplex virus (HSV-1), colds sores are incredibly common. Nearly four billion people are infected with the virus globally.i While some people experience recurring cold sores, others never show any symptoms. Read our guide to discover what a cold sore is, what it looks like and what you can do about them.