Conjunctivitis is the inflammation of the conjunctiva, a thin, delicate membrane that covers the eyeball and lines the eyelid. Conjunctivitis is an extremely common eye disease because the conjunctiva is continually exposed to microorganisms and environmental agents that can cause infections or allergic reactions.onjunctivitis may be caused by a viral infection, such as a cold; acute respiratory infection; or other disease such as measles, herpes simplex, or herpes zoster. Symptoms include mild to severe discomfort in one or both eyes; redness; swelling of the eyelids; and watery, yellow, or green discharge.