10 Causes and Treatments for Cold Sores

Cause: Stress

So what if you don't have a cold or fever, you haven't been out in the sun all day and yet you still get a fever blister? Just the thought could be stressing you out.

But wait, don't get stressed. Really. Stress is a trigger for folks who have HSV-1 [source: McKinley].

It's not stress in particular that causes the outbreak, but what stress does to the human body. Like getting sick, too much stress wears down the immune system, which in turn opens the door to an active lesion.