A cold sore is a small, fluid-filled blister that typically appears on or around the lips. It is caused by the herpes simplex virus (HSV), most commonly HSV-1. Cold sores are contagious and can be spread through close contact, such as kissing or sharing utensils. Symptoms usually include a tingling or burning sensation before the blister appears. The sores usually heal within 1 to 2 weeks without treatment. While there is no cure for the virus, antiviral medications can help reduce the severity and duration of outbreaks.