Cataract is a clouding of the lens in the eye that affects vision. For people who have cataracts, seeing through cloudy lenses is a bit like looking through a frosty or fogged-up window. The lens is made up of approximately 35% protein and 65% water. As people age, degenerative changes in the lens's proteins occur. Over time these changes cause clouding of the lens, making images look blurred or fuzzy. Most cataracts are related to aging. Cataracts are very common in older people. Cataracts are the most common cause of vision loss in people over age 40 and the principal cause of blindness in the world. Cataract surgery improves vision in up to 90% of patients.