They may not be common, but a whole lot of people are affected by rare diseases.

A disease is considered rare in the U.S. when fewer than 200,000 people have it at any given time and in Europe when it affects fewer than 1 in 2,000 people.

In the EU and U.S. combined, as many as 60 million people may experience one — because there are around 7,000 known rare diseases.

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