Cold sores are usually caused by the herpes simplex virus type 1 (HSV-1).
In most cases, the virus is passed on in early childhood, for example, when a child is kissed by a family member or friend with a cold sore.
The virus passes through the skin and travels up the nerves where it lies dormant (inactive) until it is triggered at a later date.
Cold sore triggers
Factors thought to trigger outbreaks of cold sores include:
- emotional upset or psychological stress
- tiredness and fatigue
- an injury to the affected area
- menstruation (periods)
- strong sunlight
Occasionally, cold sores can be caused by the herpes simplex virus type 2 (HSV-2). This can happen after having oral sex with a man or woman who has genital herpes, which is usually caused by HSV-2. In genital herpes, painful blisters develop on your genitals and the surrounding area.
