Glaucoma is a major cause of blindness around the world, especially in developing countries. The World Health Organization says glaucoma is a greater public health challenge than cataracts , because the blindness caused by the latter can be cured after operations. Glaucoma is a disease that makes people around the world lose their sight, and they usually don't even know they have the disease until it has permanently destroyed at least 40 percent of their sight. The process is usually so painless and subtle , so people don't notice it. Actually, the simplest medical test can discover it. "It is the leading cause of blindness in the United States. In Hispanics and in African-Americans, it's the second leading cause of blindness and so is the case in the Chinese and Indians." said Dr. Alan Robin, a specialist in treating glaucoma. "The glaucoma we see in sub-Saharan Africa is a much more aggressive blinding disease than the glaucoma we see in Americans or even African- Americans in-the United States." he said.     . Dr. Eric Fleischer also sees at Medstar Washington Hospital Center. "Pretty much anybody who has ancestors in Africa has an increased chance of developing glaucoma." Age is another thing that may lead to glaucoma, although people of all ages can get it. Glaucoma is a group of diseases that commonly produce pressure in the eye. An eye is sort of like a watch. And behind the face of the watch, liquid is made. It goes through your pupil and into the front of the watch between the face and the crystal.There's an area around the edge of the watch that drains the liquid. When that drain is blocked, the liquid can't leave the eye as fast as it is produced. The rising pressure within the eye damages and eventually kills the optic  nerve. The result is blindness. Fortunately, if caught early, glaucoma can be controlled. Glaucoma is not curable. But as  researchers worldwide lean more about it, they grow more hopeful that glaucoma can one day be cured or even prevented. From the passage, we can learn that,    _    .
It is hard for medical tests to discover glaucoma People in developed countries hardly develop glaucoma Glaucoma only causes blindness among old people The research on glaucoma is still going on worldwide