The exact definition of what makes a disease rare can vary. In the U.S., for example, rare disease is typically defined as any disease affecting less than 200,000 people, or around one in every 1,500 Americans.

A disease that affects fewer than 200,000 people in the United States. There are about 7,000 rare diseases. An estimated 25 million to 30 million Americans are living with a rare disease.

Orphanet together with the European Bioinformatics Institute jointly developed the Orphanet Rare Disease Ontology (ORDO) capturing the relationships between rare diseases, genes and other related information. ORDO also contains links to other biomedical ontologies, databases, and classification systems.