The usual cause of one or both eyes turning pink or red is conjunctivitis. Conjunctivitis, also known as "pink eye," is inflammation (redness and swelling) of the conjunctiva, the tissue covering the "whites" of the eyes and the inside of the eyelids.
The most common cause is an infection from germs (bacteria or viruses) that are passed from person to person. However, conjunctivitis also can be caused by an allergic reaction to something (for example, tree pollen), by contact with something irritating (for example, smoke in the air or chlorine in a pool), or rarely, by problems of the eye.
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