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 are the causes of rare diseases?

80% of rare diseases have a genetic origin: they are caused by an anomaly in a gene (for example cystic fibrosis) or a chromosome (for example trisomies). 

Rare diseases can also have the following origins:

  • infectious (bacterial and viral)
  • metabolic
  • environmental
  • auto-immune
  • unknown.

DID YOU KNOW? Some rare diseases have also become rare due to prenatal screening, which has had an impact on the number of births.

Genetic diseases 

Most genetic diseases are rare diseases.

Certain genetic diseases are evident from birth, such as Down's syndrome and cri du chat syndrome. Others manifest in adulthood such as Huntington's disease.

Certain genetic diseases can be hereditary, which means the anomaly that is present in the DNA is transmitted to other members of the family.

Other genetic diseases are acquired: they can result in a genetic anomaly that has arisen during a person's life (for example certain cancers).

If genetic diseases are clearly identified in the family, genetic testing can be carried out in a centre for human genetics.

Certain rare diseases can be tested for before, during or after pregnancy according to the doctor's advice.

There is systematic screening for some diseases before or at birth. 


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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