The tissue that covers the white part of the eyeball is called the conjunctiva. Pink eye happens when the conjunctiva appears reddish due to blood vessels becoming inflamed; hence the medical name conjunctivitis. While the white part of the eye is often the most affected, the inside of the eyelid, which is covered by the same tissue, may also be inflamed. The infection starts from one eye and may spread to the other so that both eyes become affected.