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Medical school students are often trained to look for the most common possible cause for a symptom; the saying goes, “When you hear hoofbeats, think of horses, not zebras.” This illustrates why rare diseases are so difficult to diagnose, and has prompted the rare disease community to adopt the zebra as its mascot.

What is a rare disease?

In the United States, any disease that affects fewer than 200,000 people is considered rare. In the EU, a rare disease is defined as affecting less than 1 in 2,000 people.

Did you know…


Approximate number of identified rare diseases, but less than 10% have an FDA-approved treatment


of people affected by a rare disease are children

Many rare diseases result in premature deaths of infants and young children, or are fatal in early childhood

6-8 years

Average number of years people with a rare disease search for a diagnosis

Rare diseases not only affect the person diagnosed – they also impact families, friends, caregivers and society as a whole.