In most cases, a fever blister tends to last anywhere between 1 and 2 weeks. They often appear in groups and lead to sore, swollen, or red wounds on your face or genitals. After a few days since the first signs, fever blisters would release a lucid fluid that might scab after a couple of days. It can be the most contagious during this period. Oral and sexual contact can spread the virus from person to person. Even when the blisters are not visible, it is still contagious. After 10 to 14 days, all symptoms might disappear on their own.