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What is a rare disease?

A rare disease is any disease that affects a small percentage of the population. There is no single, widely accepted definition for rare diseases. In Europe, a disease is considered to be rare when it affects 1 person per 2000. A disease can be rare in one region, but common in another. In the U.S., a rare disease is defined by The Orphan Drug Act as a disease or condition that impacts less than 200,000 people at any given time. Other countries may have their own official definitions of a rare disease. There is still no definition for a rare disease in Senegal.
How many rare diseases are there?

It is often said that the number of rare diseases is ~7,000, or between 5,000 and 8,000 depending on the source. However, more recently, and thank you to significant efforts from the rare disease community, and the use of resources such as OMIM [1], Orphanet [2], and MONDO [3], the number of rare diseases has been estimated higher at ~10,500 [4]. Furthermore, some studies suggest that the total number of rare diseases may be much higher if we consider the ultra-rare and hyper-rare (<1/108 individuals) diseases [5]. The variability in the estimation on the number of rare diseases is likely due to inconsistencies in inclusion/exclusion criteria, the use of different terminology, and variation in incidence in different geographical regions. In the future, as we progress towards harmonizing disease definitions across the world, we will improve our insight on the total number of rare diseases.
What is the nature of rare diseases and what are some examples?

Approximately 80% of rare diseases are genetic in origin. Others are the result of very rare forms of infections diseases, autoimmune disorders, rare cancers, and environmental causes. Rare diseases are often severe, chronic and progressive. Symptoms are often observed a birth or early in life, but some may present in adulthood. Some well known examples of rare diseases include cystic fibrosis, neuromuscular diseases such as spinal muscular atrophy and Duchenne muscular dystrophy, and neurogenetic diseases such us Huntingtons while other rarer diseases are less known.
How many people are affected by rare diseases?

Rare diseases are individually rare but common as a group. It is thought that there are over 300 million people affected by rare diseases in the world [6]. This means that if all people with a rare disease would live in the same country, this would be the third most populous country in the world. The incidence and prevalence of some diseases may change depending on the geographical area.


1. Hamosh A, Scott AF, Amberger JS, Bocchini CA, McKusick VA. Online Mendelian Inheritance in Man (OMIM), a knowledgebase of human genes and genetic disorders. Nucleic Acids Res. 2005 Jan 1;33

2. INSERM US14 (Institut national de la santé et de la recherche médicale), 96, rue Didot, 75014 Paris, France. Editor in Chief of the publication: Ana Rath, Director of Service N° 14, INSERM.

3. Mondo Disease Ontology.

4. Vasilevsky NA, Matentzoglu NA, Toro S, et al. Mondo: Unifying diseases for the world, by the world. medRxiv; 2022.

5. Smith CIE, Bergman P, Hagey DW. Estimating the number of diseases - the concept of rare, ultra-rare, and hyper-rare. iScience. 2022 Jul 1;25(8):104698.

6. What are some examples of rare diseases? Rare diseases are present across the medical spectrum. 

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