Cold sores are caused by a virus called the Herpes Simplex Virus that can sores on the lips or in the mouth. This virus can enter the body through a break in the skin around or inside the mouth and can spread to other areas of the body.
Cold sores are contagious because the virus can spread from person-to-person. For example, a person can get the virus from kissing someone infected with the virus or coming into contact with apparently normal skin that is shedding the virus.
Though a person infected with the herpes simplex virus can spread the virus to another person regardless if (visible) symptoms are present, infected individuals are the most contagious when they have “active blister-like sores.” The risk of spreading the virus is greatly decreased as the blisters dry out and form crusts (within the next few days). It is still possible for virus transmission when symptoms are not present because the virus can sometimes reside in saliva and be transmitted via contact with the infected person’s saliva. Do note that it is almost impossible to catch herpes (in this case, cold sores) from surfaces, towels, or washcloths.
After a person is infected with the Herpes Simplex Virus that caused the cold sore, the virus enters the person’s body and stays “dormant” (goes into a “deep sleep” and remains inactive) until one of the following conditions trigger the virus to become active and to replicate again causing cold sores:
- A fever, cold, or the flu
- Ultraviolet radiation (too much sun exposure)
- Stress
- Changes in the immune system
- Trauma to the involved area
- At times there may be no apparent cause of the recurrence.
It is still not understood why these condition cause the virus to be active again. No matter where the herpes infections occur first, they tend to recur in around the same area, these recurrences may happen as often as several times per year or only occasionally (once or twice a year).
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