Introduction to a Supplement on Innovative Approaches to Studying Health Outcomes in Rare Diseases

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Over the past decade, rare diseases have assumed an increasingly prominent place in health care discourse in the US and around the world. Rare diseases currently comprise approximately 6,000 different disease types, affecting about 6–8 % of people,1 or about the same proportion of the US population with diabetes.2 However, the number of rare diseases continues to rise, as new genomic data have led physicians and scientists to recognize that many diseases thought to be single entities may actually be made up of numerous genetically unique rare subtypes that have different responses to therapy and varying prognoses. In addition, drugs for rare diseases make up the largest growing category of new molecular entities approved by regulators in the US and EU.3,4New legislative programs in the US and EU have sought to encourage greater investment in research for certain types of rare diseases, including neglected tropical diseases.5 At the same time, investment by charitable foundations and p