Conjunctivitis, also known as pink eye, is the inflammation of the conjunctiva (the outermost layer of the eye that also lines the inside of the eyelids). Inflammation can occur for a variety of reasons which include a reaction to environmental irritants and viral or bacterial infections.

Both eyes or someones only one eye can be inflamed. If conjunctivitis is found in one eye, it is usually caused by an inflamed tear sac, dry eye, a foreign object. If conjunctivitis is in both eyes it is usually due to viral or bacterial infection, or a reaction to environmental irritants.