A cataract is a clouding that appears on the lens of the eye, leading to a loss in vision. Cataracts can vary in size and thus in the effect they have on vision. They can occur at any age, although they most commonly affect vision in people over the age of 60. What is known is that they result from a changing chemical balance in the lens of the eye – the part of the eye that sends light to the retina at the back of the eye. When the lens becomes cloudy, it becomes difficult to see properly. Advancing age, eye injuries, some diseases (e.g., diabetes), certain medications, smoking, and a family history of cataracts are all believed to play a role in the development of this condition. Cataracts do not spread from one eye to the other – a cataract in one eye will not cause a cataract in the other. Unfortunately, though, cataracts often develop in both eyes at the same time – although they may progress at different rates.