There are hundreds of rare diseases, disorders, and ailments; the precise number is impossible to determine because it depends on the definitions of both what qualifies as a clinical entity and what the threshold for rarity is.

In some parts of the world, policies that encourage industry investment in the development of treatments for rare diseases are used to determine the threshold for rarity.

Until recently there was no systematic effort to establish an inventory of rare disorders(diseases).

This prevented clinical research from being done and made it difficult to determine and recognise their importance for healthcare planning and budget allocation. 

This led to a lack of knowledge of their epidemiology and poor comprehension of their natural history.

In healthcare coding systems, genetic illnesses and other rare diseases had long been underrepresented because of their individual rarity.

Nonetheless, OrphaNet, first established in 1997, not only collected information on rare diseases published in the scientific literature but also classified each clinical entity being assigned an Orpha number.

Today, OrphaNet has become the reference source of information on rare diseases, providing high-quality information on rare diseases and expertise.

The World Health Organisation Rare Diseases Topic Advisory Group (TAG) was established in April 2007, to ensure that rare diseases would soon be traceable in mortality and morbidity information systems.

Orphanet was tasked with creating the fundamental data that would serve as the foundation for the ICD-11 classification of rare diseases. Given that rare diseases affect many facets of medicine, it helped with the entire ICD revision process.

The following slides show the progress toward the use of ICD-11 in the medical coding of rare diseases.