Conjunctivitis, inflammation of the conjunctiva, a mucous membrane that lines the inner surface of the eyelids and joins with the cornea of the eyeball. Conjunctivitis may be caused by infection, allergy, or injury and is marked by redness and swelling, accompanied by a feeling of roughness as the membrane passes over the exposed surface of the eyeball. The eye may be sensitive to light; in more severe cases a mucoid sticky fluid, or even pus, may be discharged, depending on the cause of the infection.

 The acute form of conjunctivitis is commonly called pinkeye. It can be caused by either bacterial or viral infection and is often epidemic. In newborn babies it may result from several kinds of cocci, especially the gonococcus (gonorrheal conjunctivitis), or from a strain of the parasitic bacterium Chlamydia trachomatis (inclusive conjunctivitis). A strain of fly-borne Chlamydia trachomatis causes trachoma, a severe form of conjunctivitis found mostly in Africa and Asia. As in gonorrheal and inclusive conjunctivitis, trachoma often results in blindness because of the involvement of the cornea. Conjunctivitis caused by exposure of the eye to a welding arc or other source of ultraviolet rays is called flash kerato-conjunctivitis.

 Most cases of bacterial conjunctivitis are treated successfully with antibiotics and sulfonamides. Viral conjunctivitis usually lasts about two weeks; treatment may be necessary in cases that involve complications. Silver nitrate or antimicrobial ointments are applied to the eyes of newborn babies to prevent ophthalmia neonatorum.