I have this problem since 10 yrs ago. I usually have it around 1-2 times a year....what makes me angry is they usually appear during holidays, such as X'mas or new year.
I read lots of information. It's a kind of 過瀘性病毒 (由疱疹I形病毒引起的), and it's 潛伏在身體神經細胞核內, therefore it's very hard (almost impossible) to find a medicine to cure it totally. When the body's immune system is low, very stressful, lack of certain vitamin...they will appear.