What causes cold sores and fever blisters in the mouth and lips?

Virus. Cold sores are caused by the herpes virus and is a very common infection to get. It is estimated that 90+% of population has been exposed to this virus in childhood. Usually it is triggered by stress, lack of sleep, excess sunlight etc.. Sometimes people get them recurrently or once in awhile or sometimes once then never again. It depends but can be treated by Acyclovir and related drugs too.
Virus. Cold sores are usually caused by hsv (type i or ii) but cold sores are usually cause by type i. Cold sores are contagious and recurrence can be cause by uv light, stress, hormone fluctuation, to fever and sometimes, there is no apparent cause. Go to your doctor for appropriate treatment options such as anti-viral medications.