Please explain what does the herpes virus do and can it be fatal?

Herpes virus. Herpes is a nasty virus that after a primary infection (cold sores or genital sores) will 'hide' in the nerve roots forevermore. It may come out again -particularly at times of stress or other disease/illness. These are 'outbreaks'. Extremely rarely, the cold sore virus (HSV-1) might go backwards into the brain, rather than forward to the lips/mouth -and in that RARE case it can be quite dangerous.