This is a regular conjunctivitis, caused by the same Herpes simplex type-1 that causes cold sores on the lip (not the STD Herpes type-2). After first entering the body, usually in childhood, Herpes simplex virus hides in a nerve center (ganglion) in the face. For various reasons it can migrate down a nerve, usually to the lip, but sometimes the eye (or even brain).
We diagnose Herpes Conjunctivitis if we see tiny blisters on the eyelids. We’re helped with our diagnosis if there’s a history of the patient having had recurrent episodes. Herpes in the eye is only dangerous if the virus invades the cornea, which causes pain (see Keratitis). That requires special oral anti-Herpes medication, not just drops; Herpes Keratitis can be blinding.