Fever blisters are caused by the herpes virus as previously stated by several people here. The trolls that keep voting down these appropriate and informed answers may be angry because they are suffering from them and don't want to be reminded of having the herpes virus in any way.
This virus already infects an estimated 3 out of 5 people of ALL ages and can lie dormant for very long periods of time. This virus is easily passed when a person is shedding the cells containing the virus and this is NOT only when the person has visible blisters.
Stress, illness, and sun exposure are some of the most common triggers for a fever blister. There are over the counter lip balms and creams that may help reduce the amount of time the fever blister is visible but the disease itself never goes away...it just lies dormant.
Lemon balm, Carmex, Abreva, are some popular choices for relief. Visiting a doctor is a good bet if you have fever blisters that won't go away or cause you an excessive amount of pain. They can prescribe medications for fever blisters and pain if needed.
Fever blisters are a COMMON problem with a few options for relief. Once a person finds what works best for them, it's probably a good idea to keep it on hand for future use.