rare diseases

A rare disease is disease that affects very few people; they very often have genetic causes. The exact definition of 'rare' varies with 1 in 2,000 being used in Europe and 1 in 200,000 in the US. However, while the prevalence of each rare disease is low, there are very many of them, over 7000 according to UK definitions. This means, paradoxically, that a substantial proportion of the population suffers from a 'rare' disease.

From a statistical point of view care needs to be taken when studying rare diseases (or any rare pheomenon) as a random sample of the population will miss nearly all rare diseases; instead some form of stratified sample is usually required. The low base rate also means that diagnostic tests will suggest other more common diseases, misdiagnosing rare ones.