Herpes Simplex or HSV keratitis is a common cause of red eye in a broad range of age groups including children, as well as the elderly. Most commonly, it affects one eye, but can be bilateral in 10% of cases. It is most commonly due to HSV-1.

Microbial keratitis requires urgent referral to an ophthalmologist. Delay in presentation (of as little as a few hours) can result in corneal perforation requiring surgical management.

Viral conjunctivitis is a self-limiting condition, and the commonest cause of a red eye presentation in general practice. It is typically caused by adenovirus. Other, less common viruses include herpes simplex, herpes zoster, enterovirus, and coxsackie virus.

Herpes Zoster Ophthalmicus (HZO) is defined by involvement of the ophthalmic branch of the trigeminal nerve (CN V1). This branch supplies the forehead, the anterior scalp, the side and tip of the nose, ethmoidal sinuses, and importantly, all structures within the globe, including the retina and optic nerve. Zoster involving the maxillary division of the trigeminal nerve (CN V2) is not globe threatening, and does not require referral to an ophthalmologist.