Cold sores are caused by a virus called herpes simplex 1 (HSV1) and herpes simplex 2 (HSV2). (Herpes simplex viruses also cause genital herpes.) Two-thirds of the adult population worldwide have been infected with HSV1. HSV1 often spreads through contact with skin or saliva from the infected person. Most people who have HSV1 got it during childhood from contact such as receiving a kiss from a relative, i.e. normally from oral to oral contact.

After someone has contracted the virus, it remains inactive (dormant) most of the time in parts of our nerves called the ganglia.

From time to time, the virus may be reactivated by certain triggers, resulting in another outbreak of cold sores. These triggers might include being run down, fatigue, hormonal changes, or sunlight.

What activates the virus to cause a cold sore? No one really understands why the virus in the ganglion suddenly wakes up from time to time and decides to go to the skin and cause a cold sore. But, in some people, there are some definite triggers that make this happen, such as:

  • bright sunshine
  • wind
  • other damage to the skin (such as ‘facial resurfacing’ procedures)
  • emotional stress
  • physical stress, such as having another illness
  • major dentistry, such as having a tooth removed
  • menstruation.


First published on:
Reviewed and edited by: Dr Anna Cantlay
Last updated: October 2020

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