Just like any machine, different body parts sometimes go wrong from old age. In fact, in the U.S., in 1998, between sixty and seventy percent of all visitors to the ER were aged seventy-five or over. Fortunately, older people who suffer from an illness or injury can now receive treatment to help extend their lives. Due to the increase in the population of elderly people, gerontology is now one of the fastest growing areas of medicine. It is now common for older people with damaged joints, for example, to have surgery to replace the old joint with a new one made of plastic or metal.