Rare diseases are those that affect less than one person in 2000. While an individual disease might be labeled as "rare", the total number of persons in Europe suffering from one of the over 7000 different identified rare diseases is estimated at over 30 million. There are approximately 30.000 individuals concerned by rare diseases in Luxembourg.

Nearly 80% of rare diseases are of genetic origin. They are present throughout a person's entire life, even if symptoms do not immediately appear. The remaining 20% are the result of infections (bacterial or viral), allergies and environmental causes.

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.

Rare diseases are often chronic and life-threatening. Most rare diseases have no cure, so living with a rare disease is an ongoing learning experience for patients and their families.