Herpes simplex virus type 1 is best known as the culprit behind cold sores. When it’s not causing itchy, crusty sores on the mouth, it hides in the bundle of nerves that run through the face. And it’s super common. An estimated 50 to 90 percent of people harbor lifelong infections of HSV-1—largely without incident.

But in some cases, HSV-1 can run through that bundles of nerves in the face and erupt in the eye. Or maybe it gets into the eye from the outside. No one is really sure. In any case, HSV-1 can definitely infect the eye. (In case you’re wondering: HSV-1 can also cause genital infections, though it’s herpes simplex virus type 2 that is more commonly associated with genital herpes.)

Herpes in the eye is as bad as it sounds. The virus infects the cornea, forming tree-like branching ulcers across the eye. It can cause irritation, pain, sensitivity to light, and ultimately blindness if untreated. Herpes in the eye is a leading cause of blindness in the world. The cornea, after all, is a transparent layer of tissue at the front of the eye, whose job is essentially to let in as much light as possible. Infection can turn the cornea cloudy—permanently.

In most cases, herpes infections in the eye can be treated with antiviral drugs like Zovirax, but the herpes viruses are becoming increasingly resistant to these drugs. Scarred corneas can also be replaced with a transplant, but past infection makes the eye more likely to reject the new tissue. Plus, once you get herpes in the eye, it can keep coming back—just like cold sores. “For some patients that experience this, there’s absolutely nothing we do,” says Dan Carr, an HSV-1 researcher at the University of Oklahoma. “Essentially they’re going to go blind if something else doesn’t happen”—if new treatments don’t become available.