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.
What Triggers an Outbreak?
Even if the virus is inactive, certain factors can wake it up and cause symptoms:
