The Unhealthy Cornea

A cornea can become unhealthy in several ways:

Injury to the cornea can result in scarring. A scar, which is dense and white, can directly impede the passage of light through the cornea. Further, a scar can cause the shape of the cornea to become abnormal and warped, which can cause incoming light to be bent in an irregular fashion as it passes through the cornea.
Warping of the cornea can also occur in certain diseases, such as keratoconus, where portions of the cornea become abnormally thin or irregularly shaped.
Disease or injury can also cause the cornea to become swollen. When the cornea swells, it also becomes cloudy, impeding the clear passage of light.
Though a cornea may otherwise be clear and regularly shaped, it must work in concert with the crystalline lens of the eye to focus light crisply on the retina. If these two structures are not correctly balanced, refractive error occurs. As such, a cornea could be too flat or steep for an eye and its lens, resulting in hyperopic or myopic refractive error and blurry vision without the use of glasses or contacts.