My BF, who is a right hypochondriac, swear by Lysine - he says they keep his cold sores at bay and boost his immune system. He's been taking them for around 9 months now & has not had his usual round of herpes (oral.....), colds or sore throats.
It helps in the production of essential amino acids or something - he initially took them for joint protection - the other benefits he discovered accidentally. Though don't take his word as gospel - he has some strange health beliefs....!
I started on them a month ago. I have just developed a cold sore..... :-(