Rare diseases

The term “rare diseases” is used to refer to diseases that occur in a relatively small number of people. Rare diseases present a complex and often overlooked challenge in healthcare. They often involve specific problems because of the fact that they are rare.

In Europe, a disease is classified as rare if it occurs in no more than 1 in 2,000 people. Around 6,200 rare diseases have been clinically defined to date and new pathologies are regularly described in the scientific literature. These conditions, despite their low prevalence, collectively impact a significant portion of the population. The number of people living with a rare disease is estimated at 3.5-5.9%, which equates to 263-446 million persons affected worldwide at any point in time. If we transpose this figure on the Belgian population, it is equivalent to a conservative estimate of 500,000 people suffering from a rare disease in Belgium

For specific information about the different rare diseases, visit the international Orphanet website.

What is a rare disease?

A rare disease affects a limited number of people. Estimations indicate the total number of different rare diseases is situated between 6,000 and 8,000.

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. About 72% of rare diseases have a genetic origin, 75% affects children and 30% of rare disease patients die before their fifth birthday.

Orphanet is the freely accessible website to provide information on rare diseases, as well as available expertise and research in the domain. More information on the international Orphanet website.

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:

  •  Cystic fibrosis (ORPHA:586; also known as “mucoviscidosis”): responsible for respiratory and digestive problems, affecting around 1 in 5,000 newborns. A test is systematically carried out at birth to mesure trypsin dosage and if, based on the results of this first test, the disease is suspected, a second analysis (genetic test) is carried out.
  • Spinal muscular atrophy (ORPHA:70; SMA): a neuromuscular disease affecting part of the nervous system that controls voluntary muscle movement. It affects around 1 in 30,000 births in Europe. Since 2022, spinal muscular atrophy has been the subject of systematic newborn screening on a national scale in Belgium. 
  • Huntington’s disease (ORPHA:399): a neurodegenerative disease of the central nervous system characterised by involuntary movements, behavioral disorders, psychiatric disorders and dementia
  • Duchenne muscular dystrophy (ORPHA:98896): a degenerative muscle disease, with an estimated birth prevalence of 1 in 3,500 — 1 in 9,300 boys
  • Neurofibromatosis type I (ORPHA:636): a condition characterised by changes in skin coloring (pigmentation, café-au-lait spots) and the growth of tumors along nerves in skin, brain, and other parts of the body
  • Amyotrophic lateral sclerosis (OPRHA:803; ALS): a progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord
  • Phenylketonuria (ORPHA:79254): an inherited disorder that increases the levels of a substance called phenylalanine in the blood which is toxic to the developing brain. If left untreated, the disease causes mental retardation. Phenylketonuria has been the subject of systematic newborn screening on a national scale in Belgium
  • Hemophilia (ORPHA:448): a condition that causes bleeding disorders, affects 1 in 5,000 boys at birth and currently affects 1 in 12,000 people in the general population.

Creutzfeldt-Jakob disease and Progeria are well-known diseases even though they are ultra-rare. You can find more detailed information on the different rare diseases on the international Orphanet website.

Did you know? Certain rare diseases, such as cystic fibrosis are more well-known and get more attention than others (read more about the Belgian Cystic Fibrosis registry). The “Ice Bucket Challenge” launched in 2014 on social networks helped to raise awareness of ALS. The famous scientist Stephen Hawking suffers from this disease.

Difficulties posed by rare diseases

Due to their scarcity and often variable and complex symptoms, rare diseases can pose certain problems: 

  • a lack of medical and scientific knowledge
  • difficulties in obtaining a (correct) diagnosis (‘diagnostic delay’)
  • difficulties in finding relevant information and an appropriate referral to experts in the rare disease
  • difficulties in finding an appropriate treatment -if existing- and access to high quality health care, social and medical support, as well as effective cooperation between general practitioners and hospitals
  • difficulties in carrying out research and studies due to difficulties in recruitment of patients and groups of patients because of the scarcity of the disease
  • a lack of treatment leading to a decrease in quality of life and life expectancy
  • existing treatment is often unaffordable
  • difficulties in social and work-related integration and independence or autonomy
  • difficulties in finding fellow patients for support and to decrease isolation.

People who suffer from a rare disease are also more vulnerable from a psychological, social, economic and cultural point of view. These problems can be overcome through an appropriate policy. Furthermore, the lack of adequate scientific and medical knowledge means that many patients do not receive a diagnosis: their diseases remain unknown. These are the people who experience the most difficulties in obtaining appropriate support.

The Belgian Plan for Rare Diseases (available in Dutch and French) formulates various action points with the aim of finding a solution to these problems. The subsequent efforts will improve the situation for rare disease patients. Sciensano is involved in several actions stipulated in the Plan for Rare Diseases.


Sciensano is responsible for several aspects included in the Belgian plan for Rare Diseases such as keeping the Orphanet database for Belgium up-to-date and processing the data from the Central Registry of Rare Diseases. This registry aims to centralise specific data on all Belgian patients affected by a rare disease. The Belgian Genetic Test Registry has been created which functions as an inventory of genetic tests available in Belgium to increase the quality management in the Centres for Human Genetics. Sciensano also hosts several disease-specific registries.

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