Conjunctivitis is an inflammation of the conjunctiva, the thin, transparent layer of tissue that lines the inside of the eyelid and the white part of the eye. The most common cause of conjunctivitis is an infection, usually viral or bacterial, although allergies and irritants can also cause it. Viral infections, such as the common cold, are the most common cause of conjunctivitis. Bacterial infections, such as those caused by Staphylococcus aureus and Streptococcus pneumoniae, are also common causes.