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A rare disease, also referred to as an orphan disease, is any disease that affects a small percentage of the population. Most rare diseases are genetic, and thus are present throughout the person's entire life, even if symptoms do not immediately appear. Many rare diseases appear early in life, and about 30 percent of children with rare diseases will die before reaching their fifth birthday. With a single diagnosed patient only, ribose-5-phosphate isomerase deficiency is considered the rarest genetic disease. No single cutoff number has been agreed upon for which a disease is considered rare. A disease may be considered rare in one part of the world, or in a particular group of people, but still be common in another. ==Definition== There is no single, widely accepted definition for rare diseases. Some definitions rely solely on the number of people living with a disease, and other definitions include other factors, such as the existence of adequate treatments or the severity of the disease. In the United States, the Rare Diseases Act of 2002 defines ''rare disease'' strictly according to prevalence, specifically "any disease or condition that affects fewer than 200,000 people in the United States,"〔(Rare Disease Act of 2002 )〕 or about 1 in 1,500 people. This definition is essentially like that of the Orphan Drug Act of 1983, a federal law that was written to encourage research into rare diseases and possible cures. In Japan, the legal definition of a rare disease is one that affects fewer than 50,000 patients in Japan, or about 1 in 2,500 people.〔(Rare diseases: what are we talking about? )〕 However, the European Commission on Public Health defines rare diseases as "life-threatening or chronically debilitating diseases which are of such low prevalence that special combined efforts are needed to address them."〔(【引用サイトリンク】 title=Useful Information on Rare Diseases from an EU Perspective )〕 The term ''low prevalence'' is later defined as generally meaning fewer than 1 in 2,000 people. Diseases that are statistically rare, but not also life-threatening, chronically debilitating, or inadequately treated, are excluded from their definition. The definitions used in the medical literature and by national health plans are similarly divided, with definitions ranging from 1/1,000 to 1/200,000.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「rare disease」の詳細全文を読む スポンサード リンク
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