The cornea is the clear tissue at the front and center of the eye. Its clarity permits light to pass into the eye, through the pupil and on to the retina at the back of the eye. Its curvature focuses or refracts the light.
There are several conditions that can affect the cornea, including infections (such as bacterial or viral keratitis), trauma, genetic disorders, degenerative disorders, autoimmune disorders, nutrition deficiencies, allergies, dystrophies and ectatic disorders (such as keratoconus), inflammatory diseases, and growths.
Blurred vision occurs whenever a disease affects either the clarity or the curvature of the cornea. Pain, sometimes quite severe, occurs if the illness affects the nerves in and near the cornea. Trauma resulting in cuts or abrasions of the cornea may also cause pain.