Fever blisters are small sores that typically occur outside the mouth on the lips, cheeks, chin, or in the nostrils. Despite the two common names (fever blister and cold sore), you do not need a cold or fever to get a fever blister. They are caused by the herpes simplex virus, which is spread through direct contact with the blister or through indirect contact by sharing items. While there is no cure, medications can speed up healing and reduce symptoms.