Causes

Cold sores are caused by certain strains of the herpes simplex virus (HSV). HSV-1 usually causes cold sores. HSV-2 is usually responsible for genital herpes. But either type can spread to the face or genitals through close contact, such as kissing or oral sex. Shared eating utensils, razors and towels might also spread HSV-1.

Cold sores are most contagious when you have oozing blisters because the virus easily spreads through contact with infected body fluids. But you can spread the virus even if you don't have blisters. Many people who are infected with the virus that causes cold sores never develop signs and symptoms.

Once you've had an episode of herpes infection, the virus lies dormant in nerve cells in your skin and may emerge as another cold sore at the same place as before. Recurrence may be triggered by:

• Viral infection or fever
• Hormonal changes, such as those related to menstruation
• Stress
• Fatigue
• Exposure to sunlight and wind
• Changes in the immune system
• Injury to the skin