The herpes simplex virus (HSV) that causes cold sores is actually activated a day or two before you have any visible symptoms, just as it is with chicken pox and other herpes viruses. So you're contagious a few days before a cold sore appears and until after the sore heals. Some people—about one-third to one-half of those who get cold sores—experience prodromal symptoms a day or two before a cold sore appears, including a slight fever and/or tingling, stinging or itching on the lips. But many do not. Your best protection? Don’t share lipstick, lip balm, a toothbrush, a drinking glass or anything else that comes in contact with your mouth.