Cold Sores Are Caused By The Herpes Simplex Virus (HSV). There Are Two Types Of Herpes Simplex Virus: HSV-1 & HSV-2. Both Virus Types Can Cause Sores Around The Mouth & On The Genitals The First Symptoms Of Cold Sores May Include Pain Around Your Mouth And On Your Lips, A Fever, A Sore Throat, Or Swollen Glands In Your Neck Or Other Parts Of The Body. After The Blisters Appear, The Cold Sores Usually Break Open, Leak A Clear Fluid, And Then Crust Over And Disappear After Several Days To 2 Weeks. For Some People, Cold Sores Can Be Very Painful.