Cold sores are small, painful fluid-filled blisters that typically appear on or around the lips and are caused by the herpes simplex virus type 1 (HSV-1). After the initial infection, the virus remains dormant in nerve cells and can reactivate periodically, often triggered by stress, illness, sun exposure, hormonal changes, or a weakened immune system. Early symptoms may include tingling, itching, or burning sensations before blisters form, which then break open, crust over, and heal within one to two weeks. Cold sores are contagious, especially when blisters are present, and can spread through close contact such as kissing or sharing personal items. While there is no cure for HSV-1, antiviral medications can help reduce the severity, duration, and frequency of outbreaks.