The term rare applies to those diseases affecting a limited number of people with a prevalence below a given cut-off point, which is codified by the legislation of each individual country. The EU sets this threshold at 0.05% of the population, i.e. one case for every 2,000 inhabitants (in the US a disease is considered rare when it has been diagnosed in fewer than 200,000 sufferers in the population, i.e. about 0.08%).
Rare diseases create specific problems relating to their rarity, are serious, often chronic and at times progressive, and may appear at birth or during childhood.