When I was in third grade, our gym teacher was often mean to Dana, one of my classmates, because Dana had trouble doing a lot of the exercises. Later Dana's family learned that she had a progressive inherited neurological disease. By fourth grade she was in a wheelchair. By fifth grade she no longer attended school. She died when we were in sixth grade.
Her younger sister also inherited the disease (I don't know what it was) and died. Their younger brother did not inherit it.