Cold sore - Causes of cold sores

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:

• 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 is no obvious trigger for an outbreak.

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.