It is usually when we are sick, stay up late or have a high fever that causes the herpes simplex virus lurking in the nerves to grow and multiply rapidly while the body’s immune system is down, causing damage to the superficial cells of the skin mucosa and the cells to become edematous and fuse to the point of forming blisters. The blisters are painful and itchy, but the disease is self-limiting and will heal on its own within a period of time, even if it is left alone.