Disability is an inescapable element of human existence and experience. Although it is rarely acknowledged as such, it is also a fundamental aspect of human diversity. It is so, first, in the sense that, worldwide, an enormous number of people are disabled. The proportion of disabled people in different national populations varies significantly with factors such as economic development, quality and availability of health care, and the age distribution of the population. (In the United States, people with disabilities make up the population's largest minority: Census 2000 found nearly twenty percent of the population over five years of age to be affected by some sort of disability [United States, Census Bureau].) Furthermore, because of the way this minority is constituted, it is arguably more diverse than those of race, gender, class, and sexual orientation. Disabilities may affect one's senses or one's mobility; they may be static or progressive, congenital or acquired, formal (affecting the shape of the body) or functional, visible or invisible.