Overview of conjunctival and scleral disorders

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The conjunctiva is the membrane that lines the eyelid and loops back to cover the sclera (the tough white fiber layer covering the eye), right up to the edge of the cornea (the clear layer in front of the iris and pupil—see Structure and Function of the Eyes). The conjunctiva helps protect the eye by keeping small foreign objects and infection-causing microorganisms out and by contributing to the maintenance of the tear film.
The most common disorder of the conjunctiva is inflammation (conjunctivitis). There are many causes of inflammation, including

Infections by bacteria (including chlamydia), viruses, or fungi (see Infectious Conjunctivitis)
Allergic reactions (see Allergic Conjunctivitis)
Chemicals or foreign bodies in the eye
Overexposure to sunlight

Conjunctivitis tends to be relatively short-lived, but some kinds last for months or years. Long-standing conjunctivitis is often caused by chronic allergies, chronic infections (trachoma), and chronic irritation of the eye that occurs when an eyelid is turned outward (ectropion) or inward (entropion), by some eye drops, or by chronic dryness. Whatever the cause, people with conjunctivitis typically have similar symptoms, such as redness and discharge. Some types of conjunctivitis also cause itching or irritation.
The sclera is the tough, white, outer layer of the eyeball. The sclera provides the eyeball with structural strength and protects against penetration and rupture. Rarely, the sclera becomes inflamed (scleritis).
The episclera is a thin tissue layer overlying the sclera. The episclera contains tiny blood vessels that supply nutrients to the sclera. Sometimes the episclera becomes inflamed (episcleritis).