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.

Anterior uveitis (iritis) accounts for the vast majority of uveitis seen in primary care. It usually, but not always, affects only one eye.

Blepharitis is common in both men and women, and affects all age groups including children. It is particularly common amongst post-menopausal women, which might point to the role of oestrogen in tear formation.

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.