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 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.

Herpes simplex virus type 2

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.

Page last reviewed: 25/04/2016

Next review due: 01/04/2019