[Comment] The RDI–Lancet Commission on Rare Diseases: improving visibility to address health-care disparities for 400 million people

Rare diseases are complex and ubiquitous; they represent a global challenge that needs a global response. A rare disease is commonly defined as a medical condition that affects fewer than one in 2000 people.1 There are thousands of different rare diseases2 that collectively affect around one in 20 people (400 million individuals worldwide)3 and include genetic diseases, cancer, infectious diseases, poisoning, immune-related diseases, idiopathic diseases, and undetermined conditions.4 People living with a rare disease (PLWRD) and their families experience common and devastating challenges due to a unifying feature for all rare diseases: their individual low prevalence.