how fever blisters form
Fever Blisters (aka Cold Sores) 1  develop on the outside of the mouth, usually along the edge of the lips. 2  are contagious until they scab over and heal completely. 3  appear as red, swollen blisters until they break, ooze and form a crust.  generally heal within 10 1  days. caused by herpes simplex virus- 1.