Some diseases that cause red eyes include keratitis, blepharitis, scleritis and subconjunctival hemorrhage, states Mayo Clinic. Red eyes result from inflammation, causing expansion of blood vessels on the surface of the eyes. This inflammation typically occurs in the conjunctiva, which is the membrane that covers the white of the eyes.
Keratitis occurs when the tissue that covers the iris and pupil or the cornea, becomes inflamed, notes Mayo Clinic. An injury on the surface of the cornea may allow fungi or bacteria to the cornea, causing infectious keratitis. Chemicals in water may irritate and weaken the tissue of the cornea or corneal epithelium, resulting in keratitis.
Blepharitis is a condition in which the oil glands in the hair follicles of the eyelids become inflamed, reports Healthline. As of 2015, the exact cause of eyelid inflammation is unknown, but risk factors include bacterial infection, a malfunctioning oil gland and having eyelash mites.
Scleritis is an inflammation of the outer surface of the eye, says Healthline. Doctors believe that scleritis occurs from the overreaction of the immune system. Risk factors include rheumatoid arthritis and inflammatory bowel disease.
A subconjunctival hemorrhage occurs from an injury or breakdown of the small blood vessels in the conjunctiva, explains MedicineNet. This condition appears as a dark red or bright red patch on the outer protective layer of the eye, or sclera. Risk factors include coughing, sneezing, eye rubbing and weight lifting.