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 is 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% of children with rare diseases will die before reaching their fifth birthday. With only three diagnosed patients in 27 years, ribose-5-phosphate isomerase deficiency is considered the rarest known genetic disease.
No single cut-off 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 be common in another.
The US organisation Global Genes has estimated that more than 300 million people worldwide are living with one of the approximately 7,000 diseases they define as "rare" in the United States.