Rare Disease
A condition that affects a small number of people relative to the population.
Definition
A rare disease is generally defined as a condition affecting fewer than 200,000 people in the United States, or fewer than 1 in 2,000 people in Europe. Although each disease is uncommon, more than 7,000 rare diseases collectively affect hundreds of millions of people worldwide. They are frequently difficult to diagnose.
The Diagnostic Journey
A typical path patients take with Rare Disease — from first symptoms to living with the condition. Click each stage to expand.
Many invisible illnesses begin with vague, intermittent symptoms such as fatigue, pain, or changes in weight or mood. Because these signs are non-specific and can come and go, they are frequently attributed to stress or overwork, which is one reason diagnosis is so often delayed.
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Sources & References
This page draws on trusted, authoritative health resources. For medical decisions, always consult a qualified healthcare professional.
- Rare DiseasesMedlinePlus, U.S. National Library of MedicineGovernment Health Agency
- About Rare DiseasesGenetic and Rare Diseases Information Center (GARD), NIHGovernment Health Agency
- Health Topics: Trusted Consumer Health InformationMedlinePlus, U.S. National Library of MedicineGovernment Health Agency
- Health Topics A-ZMedlinePlus, U.S. National Library of MedicineGovernment Health Agency
- Health Library: Diseases & ConditionsCleveland ClinicAcademic Medical Center
Content Review
- Last Reviewed
- July 2026
- Last Updated
- July 2026
- Content Type
- Educational Documentary Companion
- Evidence Standard
- Evidence-Informed
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