cold sore 
noun
An inflamed blister in or near the mouth, caused by infection with the herpes simplex virus.If it had been herpes simplex (the cold sore virus), the blisters would have been uniform in size....
- They think that it might be possible to use a version of the cold sore virus herpes simplex to treat a particularly nasty form of brain cancer called glioma.
- Herpes simplex infections commonly cause either cold sores around the mouth or blisters in the genital area (also known as genital herpes).