Pink eye or conjunctivitis develops when the membrane that covers the inner part of the eyelids and the white of the eyes -- the conjunctiva -- becomes inflamed. The vessels in the conjunctiva dilate, which makes them more visible and turns the white part of the eye a pinkish color. Though pink eye can be irritating or painful and is quite visible, it is not usually a threat to vision.
A classic sign of conjunctivitis is pinkness or redness that develops in one or both eyes. It is very difficult to prevent the spread of conjunctivitis in an infected individual, so people with pink eye generally develop bacterial conjunctivitis first in one eye and then the other. The same is true of viral conjunctivitis. The virus usually spreads from one eye to the other by rubbing the infected eye. Pink eye can also be caused by an allergic reaction or eye irritation due to chemicals or other irritants. If the cause is allergic, both eyes are usually affected at the same time.
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