Cold sores are small blisters that develop on the lips or around the mouth. They are caused by the herpes simplex virus.

The herpes simplex virus, or "cold sore virus", is highly contagious and can be easily passed from person to person by close direct contact. After someone has contracted the virus, it remains dormant (inactive) for most of the time.

Every so often the virus can be activated by certain triggers, resulting in an outbreak of cold sores. 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