Cold sores are usually caused by the herpes simplex virus type 1 (HSV-1). Occasionally, they can also be caused by the herpes simplex virus type 2 (HSV-2).
Cold sores are usually caused by the herpes simplex virus.
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 inactive (dormant) until it's triggered at a later date.
Cold sore triggers
Factors thought to trigger outbreaks of cold sores include:
- having another infection, such as a respiratory tract infection
- having a high temperature (fever)
- emotional upset or psychological stress
- tiredness and fatigue
- an injury to the affected area
- menstruation (periods)
- strong sunlight
However, in many cases there's no obvious trigger for an outbreak.
Cold sores can happen after having oral sex with a man or woman who has genital herpes.