Common cold-sore triggers for children previously exposed to the virus include: 1  Fatigue and stress. 2  Exposure to intense sunlight, heat, cold, or dryness. 3  Injuries to or breaks in the skin. 4  Illness (i.e., cold or flu) 5  Dehydration and poor diet. 6  Fluctuating hormones (i.e., during a teen's menstrual periods, etc.)