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Did you know?
Astigmatism is the most common vision problem. It may accompany nearsightedness or farsightedness. It is usually caused by an irregularly shaped cornea, but sometimes it is the result of an irregularly shaped lens. Either type can be corrected by eyeglasses, contact lenses, or refractive surgery.
Did you know?
The horizontal fraction bar was introduced by the Arabs.
Did you know?
There are approximately 3 million unintended pregnancies in the United States each year.
Did you know?
Most childhood vaccines are 90–99% effective in preventing disease. Side effects are rarely serious.
Did you know?
Autoimmune diseases occur when the immune system destroys its own healthy tissues. When this occurs, white blood cells cannot distinguish between pathogens and normal cells.