What is a rare disease?

A rare disease, also referred to as an orphan disease, is any disease that affects a small percentage of the population.

Most rare diseases are genetic, and are present throughout a person's entire life, even if symptoms do not immediately appear. In Europe a disease or disorder is defined as rare when it affects less than 1 in 2000 citizens.

Rare diseases are characterised by a wide diversity of symptoms and signs that vary not only from disease to disease but also from patient to patient suffering from the same disease.

Relatively common symptoms can hide underlying rare diseases, leading to misdiagnosis.

Read the fact sheet "What is a rare disease?"


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While an individual disease might be labeled as “rare”, the total number of persons in Europe suffering from one of the over 6000 different identified rare diseases is estimated at over 30 million.

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

An individual rare disease may affect only one person in a million, but all together, rare disease patients comprise 6% to 8 % of the EU population.


Page created: 19/08/2009
Page last updated: 09/09/2014
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