@fify-- I'm exactly the opposite. I get fever blisters on my lips when I'm out in the sun or when I have a fever.
This is why I have to use lip balm with SPF and UV protection. When I get sick, I have to keep an eye on my temperature and take fever reducers if I get a fever. Otherwise, I will get a fever blister immediately.
I'm not sure if this is a cure or fever blister treatment but I've heard great things about lysine supplements. Apparently, this supplement helps prevent fever blisters. I'm planning on buying a bottle, you might want to look into it if you get blisters often.