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A condition is defined as rare when it affects fewer than 1 in 2,000 people, thus requiring a specific approach for diagnosis, treatment, and care coordination. In France, these conditions are a major public health issue, with over 7,000 identified rare diseases affecting approximately 3 million people, or 4.5% of the population.

Rare diseases are generally severe, chronic, and progressive. They significantly impact quality of life, with motor, sensory, or intellectual deficits in 50% of cases. The term "orphan disease" specifically refers to a rare condition with no known effective treatment.