Cataract

Source: all about vision
A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil.
Cataracts are the most common cause of vision loss in people over age 40 and is the principal cause of blindness in the world. In fact, there are more cases of cataracts worldwide than there are of glaucoma, macular degeneration and diabetic retinopathy combined, according to Prevent Blindness America (PBA).
Today, cataracts affect more than 22 million Americans age 40 and older. And as the U.S. population ages, more than 30 million Americans are expected to have cataracts by the year 2020, PBA says.


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http://www.allaboutvision.com/conditions/cataracts.htm