Dry eye that occurs when tears aren't able to provide adequate moisture. Red eyes are usually caused by allergy, eye fatigue, over-wearing contact lenses, or common eye infections such as pink eye (conjunctivitis). Red eyes can be the sign of a minor irritation or a serious medical condition, such as an infection. Red eyes happen when the tiny blood vessels on the surface of the whites of your eyes expand, turning the whites of one or both eyes a pink or. Find out when to get medical advice if you have a red eye, and read about the most common causes.