In the European Union, a disease is considered rare when it affects 5 people in 100.000 (1 person in 2,000). One of the epidemiological characteristics of these diseases is that the highest incidence occurs in the paediatric age group, since a large number of them are due to genetic and congenital causes, while their prevalence is higher in adults, due to a very high infant mortality rate for some of them. Its geographical distribution can also be highly variable.

Rare diseases are an emerging global public health priority. Given their high heterogeneity and the need to provide individualised medicine for small groups of patients, they have become a challenge for health systems and biopharmaceutical research.

According to available epidemiological data, they affect between 3.5 and 5.9 per cent of the world’s population, equivalent to about 300 million people worldwide, a figure used so far by Rare Diseases International and EURORDIS.