Introduction: hereditary hemorrhagic telangiectasia as a rare disease.

A disease is considered rare in the United States when it affects one individual per 1, 250 and one individual per 2,000 in Europe. Most rare diseases (RD) are of genetic origin; their rarity involves a difficult and/or late diagnosis. The greatest barrier to prevention, diagnosis and treatment of RD is inadequate knowledge. Hereditary haemorrhagic telangiectasia (HHT) is a "rare" genetic disorder that is becoming more commonly recognised. Recent evidence indicates that it is more frequent (1-2/10,000) than previously estimated. We suppose that the frequent misdiagnosis and the different genetic penetrance have led to an underestimation of real prevalence. In fact, progress in scientific knowledge and improvement in diagnostic and therapeutic technologies has unmasked conditions which were not fully known previously, determining a fictitious decrease in disease frequency.