What Causes Cold Sores?

The virus that causes cold sores is known as the herpes simplex virus (HSV). There are two types of HSV, type I and type II. Cold sores are usually caused by type I.

Herpes simplex is a contagious oral virus. The virus is spread from person to person by kissing or another close contact with sores or even from contact with apparently normal skin that is shedding the virus. Infected saliva is also a means of spreading the virus. The most contagious period is when a person has active blister-like sores. Once the blisters have dried and crusted over (within a few days), the risk of contagion is significantly lessened. However, a person infected with HSV can pass it on to another person even when a cold sore is not present. This is because the virus is sometimes shed in saliva even when sores are not present. Despite popular myth, it is almost impossible to catch herpes (cold sores) from contaminated surfaces, towels, or washcloths.

After the first infection, the virus enters the nerve cells and travels up the nerve until it comes to a place called a ganglion, which is a collection of nerve cells. There, it resides quietly in a stage that is called "dormant" or "latent." In more active stages, the virus starts multiplying again and travels down the nerve to the skin, causing blisters on the lips known as cold sores. The exact way this happens is not clear, but it is known that some conditions seem to be associated with recurrences, including

fever, colds, or the flu (this is why some people call them "fever blisters");

ultraviolet radiation (exposure to the sun);

stress;

changes in the immune system;

hormonal changes, such as menstruation; and

trauma to the skin.

Sometimes there is no apparent cause of the recurrence.

Cold sores have a tendency to recur in more or less the same place each time. Such recurrences may happen often (for example, once a month) or only occasionally (for example, once or twice a year).