The other virus that causes herpetic eye disease is herpes simplex type 1, or HSV1. Herpes simplex type 1 is the same virus that causes cold sores on the lips and mouth. In the eye, it usually causes an infection of the cornea called herpes simplex keratitis. Herpes simplex type 2 can cause keratitis, particularly in newborns, but less frequently.
Like many viruses, the herpes simplex 1 and varicella-zoster viruses are present in most adults, since they usually infect someone earlier in life. The viruses in the herpes family usually live around the nerve fibers in humans without ever causing a problem. Occasionally, the viruses will multiply or move from one area of the body to another, and that is when herpetic disease breaks out. Outbreaks often happens when the body's immune system is weakened by some other health problem.