A rare disease is a disease that affects a small percentage of the population. In some parts of the world, the term orphan disease describes a rare disease whose rarity results in little or no funding or research for treatments, without financial incentives from governments or other agencies. Orphan drugs are medications targeting orphan diseases.
Most rare diseases are genetic in origin 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 birthdays.[1] Fields condition is considered the rarest known genetic disease, affecting two known individuals, Catherine and Kirstie Fields. With four diagnosed patients in 27 years,[needs update?] ribose-5-phosphate isomerase deficiency is considered the second rarest.[2][3]
While no single number has been agreed upon for which a disease is considered rare, several efforts have been undertaken to estimate the number of unique rare diseases. In 2019, the Monarch Initiative released a rare disease subset of the Mondo ontology that reconciles a wide variety of rare disease knowledge sources, such as OMIM and Orphanet. This was the first count since 1983, demonstrating that there were >10,500 rare diseases where prior estimates had been ~7,000 in the Orphan Drug Act.[4] Global Genes has also estimated that currently approximately 10,000 rare diseases exist globally, with 80% of these having identified genetic origins.[5]