The herpes simplex virus remains hidden in your nerves for the rest of your life. It can become active again from time to time causing a cold sore. This will usually show up in the same places where you may have had a previous infection. Most people don't get more than one recurrence of cold sores per year, but about 5–10% of people experience 6 or more episodes per year. Situations that trigger the virus to become active include a fever (eg, a common cold), menstruation, trauma, UV radiation (exposure to sunlight), extreme tiredness or lowered immune function.