Rare diseases

Although some diseases are rare, the number of people suffering from them is significant. There are between 6,000 and 8,000 rare diseases affecting a total of 6 to 8% of Belgian people.

What is a rare disease?

A rare disease affects a limited number of people. Between 6,000 and 8,000 rare diseases have been identified to date.

In Europe:

  • a disease is said to be “rare” if it affects less than 1 in 2,000 people
  • a disease is said to be “ultra-rare” if it affects less than 1 in 50,000 people
  • a disease is said to be “orphan” when there is no existing treatment to cure it. 

A large majority of rare diseases are orphan diseases and vice-versa. 

A rare disease can be chronic, degenerative, disabling and often life-threatening. 

DID YOU KNOW? A disease may be rare in one region or in one community, but very common in another. Malaria has become a rare disease in Europe but remains endemic in Sub-Saharan Africa. Furthermore, there are lots of common diseases with rare variants.

What are the most well-known rare diseases?

At present, there are very few reliable figures on rare diseases. Nonetheless, we can list a few examples of known rare diseases:

  • spina bifida 
  • cystic fibrosis (also known as “mucoviscidosis”) 
  • Huntington’s disease 
  • neurofibromatosis type I 
  • amyotrophic lateral sclerosis (ALS)
  • Duchenne muscular dystrophy
  • phenylketonuria 

Creutzfeldt-Jakob disease and Progeria are well-known diseases even though they are ultra-rare.

DID YOU KNOW? Certain rare diseases, such as cystic fibrosis are more well-known and are given more publicity than others. The “Ice Bucket Challenge” launched in 2014 on social networks helped to raise awareness of amyotrophic lateral sclerosis. The famous scientist Stephen Hawking also suffers from this disease.

Difficulties posed by rare diseases

Due to their scarcity, rare diseases can pose certain problems: 

  • varied and complex symptoms
  • a lack of medical and scientific knowledge
  • difficulties in making a diagnosis (errant diagnosis) 
  • difficulties in finding an appropriate treatment 
  • difficulties in carrying out research and studies (difficulties in recruiting patients and groups of patients that are too small).



Sciensano is responsible for processing the data from the Central Registry of Rare Diseases, which aims to centralise certain data on all Belgian patients affected by a rare disease.

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