Rare disease
disease that affects a small percentage of the population
A rare disease is any 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. Fields condition is considered the rarest known disease, affecting three known individuals, two of whom are identical twins. With four diagnosed patients in 27 years, ribose-5-phosphate isomerase deficiency is considered the second rarest.
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.
The public source identifies “Rare disease” as disease that affects a small percentage of the population. This brief keeps that definition visible, then builds a research path around Rare, disease and affects.
Why this record matters
A short description can identify a subject without explaining its stakes. For “Rare disease”, the useful work is to connect “disease that affects a small percentage of the population” to the records capable of establishing context and consequence.
Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Aug 2, 2026. The linked authority identifier is Q929833. The Library of Congress control number is sh92005613. None of the 1 selected statements returned an explicit reference. The first chronological checks are 2019 and 1983.
A concise general-reference account can conceal disagreements about scope, terminology or the weight assigned to individual sources. The lead is largely declarative, so disagreement and counter-evidence require a deliberate search beyond the opening account. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
How to read it
Use the entry as an orientation point, then follow its citations and revision history. Names, dates and institutional relationships should be checked against the original record.
- Subject orientation
- Search vocabulary
- Locating named sources
The closest primary source, responsible institution and strongest cited specialist reference.
Three-step research path
- Establish the record: confirm the title “Rare disease”, its source revision and the description used here.
- Expand the search: follow Rare disease primary sources, Rare disease archive and Rare research across catalogues and specialist indexes.
- Test the account: compare the strongest cited source with the responsible institution’s current record and note any disagreement.
Questions for further research
- Which source most directly establishes the central claim about “Rare disease”?
- What terminology or title could unlock a more precise catalogue search?
- Which institution is responsible for the underlying evidence?
Search terms from this dossier
This entry incorporates text from “Rare disease” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.