What is a Rare Disease?

Rare diseases are diseases which affect a small number of people compared to the general population and specific issues are raised in relation to their rarity. In Europe, a disease is considered to be rare when it affects fewer than 5 people in 10.000. A disease can be rare in one region, but common in another. There are also many common diseases whose variants are rare.

How many rare diseases are there?
There are thousands of rare diseases. To date, six to seven thousand rare diseases have been discovered and new diseases are regularly described in medical literature.

What are the origins and characteristics of rare diseases?
While nearly all genetic diseases are rare diseases, not all rare diseases are genetic diseases. There are also very rare forms of infectious diseases, such as auto-immune diseases and rare cancers. To date, the cause remains unknown for many rare diseases. Rare diseases are serious, often chronic and progressive, diseases. For many rare diseases, signs may be observed at birth or in childhood. However, over 50% of rare appear during adulthood.