I cannot remember ever having a cold sore, but as most people carry the virus I'm sure it's just latent in all of us. They are caused by a strain of the herpes virus which is a nasty family including cousins like the chicken pox virus and shingles. The cold sore strain is spread by kissing so not easy to avoid. They tend to pop up in people when immunity is low. Acyclovir should heal the blister. They haven't worked out how to get rid of the virus, but there is a vaccination for shingles which has some efficacy depending on the age given.