How could a child, 10 or so, become blind and gain no physical defects?

I'm working on a story, and my character becomes blind around the age mentioned above. Most of the causes of blindness I've researched cause some physical defects, live a visible tumor or severe redness and swelling of the eye, et cetera. So, what could happen to make my character lose her sight? Some kind of disease, an accident, what? You people are smart - help me out, here! =]