Fever blisters, also called “cold sores” are caused by the type I herpes virus and usually occur on the outside of the mouth, on the lips, chin, cheeks or inside the nostrils.
These are often confused with canker sores, which occur only in the mouth. If fever blisters appear in the mouth they form on the gums or roof of the mouth, and are usually smaller than canker sores.
Cold sores are extremely contagious and are usually spread by kissing, but can also spread by touching the cold sore and then touching other people. Only about “10 percent of oral herpes infections in adults result from oral-genital sex with a person who has active genital herpes (type 2).” (National Institute of Dental and Craniofacial Research)
Interestingly, “[m]ost people infected with type 1 herpes ... became infected before they were 10 years old. The virus usually invades the moist membrane cells."