Your doctor is right; the Herpes virus related to cold sores is a very common virus. You could easily have been carrying it in your body for years and not known it or you could have been exposed to it from your current partner. Stress can certainly contribute to a weakened immune system, which can bring on an outbreak.
If your current partner does not already carry the Herpes virus, there is a chance of you passing it to her. If you are having symptoms the chance is a lot higher than if you are not but even without symptoms it could happen. One study found that people who carry type 1 herpes (this is the type that usually causes recurring sore on the mouth) had the virus around their mouth 9-18% of the time when they had no symptoms.
There are a few things that you can do to reduce the chance of passing the virus. First, if you know which strain of the virus you have, your partner can be tested to see if she already is a carrier of this strain. If she is, then she cannot be re-infected so there is no worry. If she does not carry this strain then the two of you should avoid any contact with the sores when they are present. This means no kissing or performing oral sex on her. This would also mean avoiding any contact with the area if you are noticing any warming, burning or tingling sensations around your mouth as this could be the start of an outbreak.
Check out our page on Herpes Simplex Virus for more information about this very common virus. Be sure to look at some of the resources listed at the bottom of the page.
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