A cold sore is a small, painful blister typically found on or around the lips, caused by the herpes simplex virus type 1 (HSV-1). It often begins with a tingling or burning sensation before blisters appear. These blisters can break open, ooze, and form a crust as they heal over 7 to 10 days. Cold sores are highly contagious and can spread through close contact, such as kissing or sharing utensils. Triggers like stress, illness, or sun exposure can reactivate the virus in someone who has had a previous outbreak. While there is no cure, antiviral medications can help reduce the severity and duration of symptoms.