A cataract is a clouding of the lens in the eye that affects vision. (The lens is a clear part of the eye that helps to focus light, or an image, on the retina, the light-sensitive tissue at the back of the eye.) Most cataracts are related to aging.
Cataracts are common in older people. By age 80, more than half of all Americans have either a cataract or have had cataract surgery. Cataracts also have been associated with UVB sunlight, so wearing sunglasses with complete UV protection is important. A cataract can occur in either or both eyes. It cannot spread from one eye to the other.
— Source: National Institutes of Health