A cold sore is a fluid-filled blister. It can look red and irritated. Eventually, it breaks open, oozes fluid, and forms a crust. Most cold sores are on the lips or around the mouth, but they can appear elsewhere on your face. If they are near your eyes, you should see a doctor right away. Left untreated, a cold sore near the eye could lead to a corneal infection, which could lead to blindness. If you touch a cold sore and then touch another part of your body (for example, your genitals), you can spread the virus. In healthy people, cold sores usually last five to 15 days. If your cold sore persists longer than that, you should see a dermatologist.