Pink eye (conjunctivitis) is an inflammation or infection of the transparent membrane (conjunctiva) that lines your eyelid and covers the white part of your eyeball. When small blood vessels in the conjunctiva become inflamed, they're more visible. This is what causes the whites of your eyes to appear reddish or pink. Pink eye is commonly caused by a bacterial or viral infection or an allergic reaction. It may affect one or both eyes. Pink eye can be irritating, but it rarely affects your vision. Treatments can help ease the discomfort of pink eye.