Final answer:
Cataracts are the greatest single cause of blindness in the world. They occur when the lens of the eye becomes cloudy, leading to vision loss and eventually blindness.
Step-by-step explanation:
The greatest single cause of blindness in the world is cataracts. Cataracts are responsible for 50% of all cases of blindness. They occur when the lens of the eye becomes cloudy, blocking light from entering the eye. This leads to vision loss and eventually blindness.