A rare disease is any disease that affects a small percentage of the population. In some parts of the world, an orphan disease is a rare disease whose rarity means there's a lack of a market large enough to gain support and resources for discovering treatments for it, except by the government granting economically advantageous conditions to creating and selling such treatments. Orphan drugs are ones so created or sold.

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.[1] With a single diagnosed patient only, ribose-5-phosphate isomerase deficiency is considered the rarest known 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.

Global Genes have estimated that more than 300 million people worldwide are living with one of the 7,000 diseases they define as "rare" in the United States.

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An orphan drug is a pharmaceutical agent which has been developed to treat a rare medical condition, the condition itself being referred to as a rare disease.[citation needed] A rare disease, also referred to as an "orphan disease", is any disease which 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.[5] With a single diagnosed patient only, ribose-5-phosphate isomerase deficiency is presently[when?] considered the rarest genetic disease. No single cutoff number has been agreed upon for which a disease is considered rare.[citation needed] 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.[citation needed] As many as one-in-ten Americans suffers from rare disease.[6] According to clinuvel.com, over 55 million people are estimated to suffer from a rare disease in Europe and in the US, global estimates are between 5000 and 7000 rare diseases and new rare diseases are discovered every week and many have no treatments available.

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