A cataract happens when the lens of the eye becomes cloudy and prevents light rays from clearly passing through and focusing on the retina.  It can be lenient enough that vision is barely affected, or so severe that seeing shapes or movements will not be possible. Cataracts can cause decreased vision, glare while driving at night, and halos around light, all of which affect daily activities.
Cataracts happen when certain proteins in the eye lens form into abnormal clumps. These clumps gradually enlarge and interfere with vision by distorting or blocking the passage of light through the lens. The type of cataract can be nuclear, cortical, and posterior subcapsular, determined by the layer of the lens affected. The world’s leading cause of blindness is cataracts, responsible for approximately 42 percent of all cases of blindness across the globe.
Cataracts typically show no symptoms until they have grown large enough to alter significantly with the passage of light through the lens. When cataracts develop, symptoms may include: 
- cloudy or blurry vision
- double vision
- color blindness
- seeing halos around lights
- increased sensitivity to glare.
The following are the causes of cataracts; 
In many cases, cataracts are age-related, the lens of the eye is made up of fluids that contain protein, as people age this fluid may begin to cloud over, causing cataracts. This condition is likely to appear when a person approaches his forties or fifties, but may not affect vision until after the age of 60. As age keeps adding up, the lenses in the eyes become less transparent, less flexible, and thicker. Also, Age-related medical conditions cause tissues within the lens to break down and clump together, clouding small areas within the lens.
Although, in infants, cataracts may be present since birth, occurring as a result of an infection that happened during pregnancy, most especially syphilis or herpes simplex. In infants and young children, cataracts may also be one symptom of a metabolic disease affecting the body’s processing of carbohydrates, calcium, or copper.