Herpetic simplex keratitis is a form of keratitis caused by recurrent herpes simplex virus (HSV) infection in the cornea.It begins with infection of epithelial cells on the surface of the eye and retrograde infection of nerves serving the cornea. Primary infection typically presents as swelling of the conjunctiva and eyelids (blepharoconjunctivitis), accompanied by small white itchy lesions on the corneal surface. The effect of the lesions varies, from minor damage to the epithelium (superficial punctate keratitis), to more serious consequences such as the formation of dendritic ulcers. Infection is unilateral, affecting one eye at a time. Additional symptoms include dull pain deep inside the eye, mild to acute dryness, and sinusitis. Most primary infections resolve spontaneously in a few weeks. Healing can be aided by the use of oral and topical antivirals. Subsequent recurrences may be more severe, with infected epithelial cells showing larger dendritic ulceration, and lesions forming white plaques. The epithelial layer is sloughed off as the dendritic ulcer grows, and mild inflammation (iritis) may occur in the underlying stroma of iris. Sensation loss occurs in lesional areas, producing generalised corneal anaesthesia with repeated recurrences. Recurrence can be accompanied by chronic dry eye, low grade intermittent conjunctivitis, or chronic unexplained sinusitis. Following persistent infection  the concentration of viral DNA reaches a critical limit. Antibody responses against the viral antigen expression in the stroma can trigger a massive immune response in the eye.
Hypopyon is a medical condition involving inflammatory cells in the anterior chamber of the eye. It is an exudate rich in white blood cells, seen in the anterior chamber, usually accompanied by redness of the conjunctiva and the underlying episclera. It is a sign of inflammation of the anterior uvea and iris, i.e. iritis, which is a form of anterior uveitis. The exudate settles at the dependent aspect of the eye due to gravity. It can be sterile (in bacterial corneal ulcer) or not sterile (fungal corneal ulcer). Differential diagnosis
Hypopyon can be present in a corneal ulcer. It can occur as a result of Behçets disease, endophthalmitis, panuveitis/panophthalmitis, or adverse reactions to some drugs (such as rifabutin).Hypopyon is also known as sterile pus because it occurs due to the release of toxins and not by the actual invasion of pathogens. The toxins secreted by the pathogens mediate the outpouring of leukocytes that settle in the anterior chamber of the eye. An inverse hypopyon is different from a standard hypopyon. Inverse hypopyon is seen after a pars plana vitrectomy with an insertion of silicone oil (as a replacement of the vitreous humour that has been removed in the operation; the silicone oil maintains internal tamponade). When the silicone oil emulsifies, it seeps into the anterior chamber and settles at the top of the anterior chamber. This is in contrast to hypopyon resulting from toxins where the leukocytes settle at the bottom of the anterior chamber.