Keratitis is an ocular emergency that can lead to severe visual disability and requires prompt diagnosis and treatment. Sequelae can vary in severity from little or no visual loss to corneal scarring, perforation, endophthalmitis, and loss of the eye. Although the corneal surface is awash with microorganisms of the normal flora, an intact corneal epithelium and ocular defense mechanism serve to prevent infection in the normal eye. Although some organisms such as Neisseria gonorrhoeae, Neisseria meningitidis, Corynebacterium diptheriae, Listeria, and Shigella can penetrate an intact epithelium, all others require damage to the epithelial layer to invade the cornea. Several risk factors predispose the cornea to infection. Dry eyes from Sjogren syndrome, Stevens–Johnson syndrome, or vitamin A deficiency can result in bacterial keratitis. Prolonged corneal exposure from ectropion, lagophthalmos, or proptosis can lead to secondary infection. Entropion and trichiasis resulting in epithelial defects put the cornea at risk. Neurotropic keratopathy from cranial neuropathy, prior herpes simplex, or zoster infections predispose to secondary infections. Some systemic conditions such as chronic alcoholism, severe malnutrition, immunosuppressive drug use, immunodeficiency syndromes, and malignancy can impair immune defenses and allow infection by unusual organisms. Prior ocular surgery such as penetrating keratoplasty or refractive procedures is also a risk factor. Trauma is a common predisposing factor of bacterial keratitis, especially for patients at the extremes of age and in developing countries. Injury to the corneal surface and stroma allows invasion of normal flora as well as organisms harbored by foreign bodies.

Contact lens wear is the most common established risk factor for bacterial keratitis in developed countries. All types of contact lenses have been linked to infection, with extended-wear soft lenses conferring greater risk than daily wear hard or soft lenses. Corneal changes from contact lens use include an induced hypoxic and hypercapnic state promoting epithelial cell derangement and allowing bacterial invasion. Contact lenses also induce dry eye and corneal hypesthesia. Overnight rigid gas-permeable lens use for orthokeratology has also been associated with bacterial keratitis, but with a disproportionately high incidence of Acanthamoeba keratitis.