Cold sores (or oral herpes) are a very common mouth infection caused by the herpes simplex virus. It causes small, fluid-filled blisters to develop around the lips or inside the mouth, which may also crust over. The herpes simplex virus is highly contagious, and is passed from person to person by close contact such as kissing, or by sharing an object that has been in contact with the virus such as a toothbrush or lipstick.