[Rare diseases: general principles, specific problems, and health interventions].

Rare diseases are a large group of about 5000 diseases, 90% of which are genetically determined, which potentially involve all human organs and are characterized by a low prevalence in the population; this is frequently associated to problems in achieving a rapid and correct diagnosis and an adequate treatment. Up to now, a generally accepted definition or classification of rare diseases is not available. The Programme of Community Action 1999-2003 on rare diseases within the framework for action in the field of public health gives the following definition for rare diseases: diseases with a prevalence in the European Union (EU) population of less than 5 per 10,000. Notwithstanding the low prevalence of each disorder, rare diseases are very numerous and, when considered overall, they are a significant public health problem in many countries (e.g. Italy, Europe, USA); in fact rare diseases are often chronic and invalidating or cause early mortality. In this report, we illustrate some general concepts and specific problems regarding rare diseases as well as public health activities being undertaken in the USA, EU and Italy.