A cold sore is a fluid-filled blister (or a cluster of blisters) that appears on the lips and around the mouth. Cold sores are also called fever blisters, oral herpes or herpes labialis. Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. Jan 16, · Cold sores, also known as herpes labialis, are caused by nongenital herpes simplex virus type 1. People can develop a cold sore on their lips or in their mouth. .