A cataract can be caused from trauma, or more commonly, old age. A cataract is what occurs when your optical lens in your eye becomes so dense, that it cannot allow light to filter to your retinae, effectively making your vision dim and/or blurry.
The only effective means in removing a cataract, and therefore restoring vision, is surgery.
It is believed that cataracts come with age, and that if everyone lived long enough, everyone would develop one.
After being hit with a double A battery that ricocheted off the wall, I developed a massive cataract.