Over-exposure to the sun, digestive upsets or the use of antibiotics can also cause fever blisters. I used to get horrible fever blisters - they would cover my entire mouth - and mostly when I was constipated or had to take antibiotics. One doctor told me that these things could upset the natural bacteria in your digestive system and allow the virus that causes fever blisters to flurish.
When I first start to get the burn or sting that indicated a fever blister is starting to form, I take LACTINEX (it can be purchased at the pharmacy without a prescription (it's kept in the refrigerator). I chew 4 tablets to begin with, then 4 tablets every four hours for a couple of days, and they usually don't even develop. The reason it is kept in the refrigerator is that if replaces the good bacteria in your system and the cold prohibits the growth of the bacteria until you ingest it.
I very seldom even get them anymore, and I'm not lllying when I say I used to get horrible ones -- my junior year in high school, starting at a new high school, they were so bad my lips turned inside out - I looked like a Ubangi! It would take two weeks to heal! Needless to say, I really impressed my new classmates. NOT!