Cold sores are caused by the herpes simplex virus, most often HSV-1, which stays in your nerve cells for life after the first infection and reactivates from time to time.
You catch the virus through direct contact with saliva, skin, or an active sore, and many people are exposed in childhood without ever noticing. Once you have it, outbreaks tend to return in the same spot on or near the lip because the virus travels back down the same nerve. Sunlight, fever or another illness, stress, poor sleep, lip injury, and hormone changes are the usual triggers. Antiviral creams and tablets work best when you start them at the first tingle, before a blister forms. Daily sunscreen or SPF lip balm helps if sun sets off your outbreaks.
How to prevent cold sores
Common questions
What causes cold sores?
Cold sores come from herpes simplex virus, most often HSV-1 and sometimes HSV-2. The virus spreads through direct contact with saliva, skin, or sores, including kissing and sharing cups, utensils, razors, towels, or lip balm. After the first infection it stays inactive in nerve cells near your jaw and can reactivate later, which is why sores keep coming back in the same place. Common triggers include fever or another illness, strong sunlight on the lips, cold wind, stress, poor sleep, injury to the lip, and hormone changes around your period. Sores are most contagious while blisters are open, and antiviral treatment helps most when started at the first tingle.