It’s probably happened to you, your child, or someone you know: you wake up one morning with a swollen, crusty, runny, and irritated eye, seemingly out of nowhere. Pink eye, medically known as conjunctivitis, is one of the most common eye conditions, with between three and six million cases occurring in the U.S. each year.

Pink eye is defined as inflammation of the thin transparent tissue (conjunctiva) that covers the outer surface of the eye and the inner surface of the eyelid.