Reactivation of the varicella virus. Basically when one gets Chickenpox the body overcomes it, but the virus remains on the nerves. This is true of herpes. It reactivates when the immune system is weak. With herpes, people get cold sores when they are stressed or have a cold. Thus "cold sore". I notice more people with cold sores before exams. I'm evil like that.

It's rare, but sometimes herpes can travel the opposite way up the facial VII nerve and cause problems in the brain.

Apparently I've been under a lot of stress lately?