What is a rare disease?

The definition of a rare disease differs between the United States and the Europe.

In the United States (US), a rare disease is a disease that affects fewer than 200,000 persons. So, theoretically, a disease that affects 199,999 people is considered as rare in the US.

In the European Union (EU), a rare disease is a disease that affects fewer than 1 in 2,000 people. Compared to the US, this means that a disease should affect fewer than 200 people in 200,000 to be considered as rare in the EU.

Thus, rare disease defining criteria appear to be more stringent in Europe than in the US.

An ultra-rare disease is a disease that affects less than 1 per 50,000 persons.