Cold sores—also called fever blisters—are caused by herpes simplex virus type 1 (HSV-1). While they’re usually harmless, they’re contagious, often painful, and tend to reappear once you’ve had your first outbreak. That’s because HSV-1 never fully leaves—it hides in your nerve cells and flares up when triggered.

What Causes Cold Sores?
Most people catch HSV-1 in childhood, often through kissing or sharing utensils, towels, or lip balm. After the initial infection, the virus goes dormant but never disappears, lurking quietly until something reactivates it.

HSV-1 typically causes oral cold sores, whereas HSV-2 is more often linked to genital outbreaks—but either type can affect multiple areas depending on how the virus spreads.