Rare diseases: what you need to know

What is a rare disease?

A rare disease is a disease that a small number of people have. Rare diseases affect fewer than 1 in 2000 people. Below are some types of rare diseases.

• Metabolic disorders. These disrupts how your body breaks down food or converts it into energy (your metabolism).
• Musculoskeletal conditions. They affect bones, joints or muscles.
• Neurological conditions. These conditions affect the brain, spinal cord or nerves, such as Huntington’s disease.
• Infectious diseases. Rare infections caused by uncommon bacteria, viruses or parasites.
• Some rare cancers. Cancers that occur infrequently or involve unusual cell types or locations in the body.
• Immunodeficiencies. Conditions where the immune system doesn’t work properly, making people more vulnerable to infections.

There are about 7,000 known rare diseases and more continue to be found.

What causes a rare disease?

Around 80 per cent of rare diseases are a result of genetics. This means a change (variant) in a person’s DNA is causing the disease. Rare diseases can run in families.

Other rare diseases are not genetic. They may be caused by environmental factors such as:

• diet
• smoking
• exposure to chemicals

Some conditions are caused by a combination of both genetic and environmental factors.

How are rare diseases diagnosed?

Getting a diagnosis for a rare disease can take time. For many people, it can take around five years from the first symptoms to receiving a diagnosis.

This delay can happen because:

• symptoms may be unclear or overlap with other conditions
• you may need to see different specialists
• you had a misdiagnosis, where you get diagnosed with the wrong condition

Genetic testing may be involved in the diagnosis process. Many rare diseases have a genetic cause. When you see a GP and they think your symptoms have a genetic cause, they can refer you for genetic testing.

Genetic testing (also called genomics testing) involves analysing a person’s DNA, usually from a blood or saliva sample, to look for changes (variants) in genes that may explain their symptoms. If a rare disease is caused by a known genetic change, testing can help identify the specific gene involved and confirm the diagnosis.

How are rare diseases treated?

Unfortunately, most rare diseases don’t have an approved treatment. The rare diseases that do have treatments are mainly cancers.

Having a rare disease can mean having very complex needs. Medicines can be developed for rare diseases which are called orphan drugs. Access to treatment can however be difficult. Some people with rare diseases may be able to take part in clinical trials, which test new or experimental treatments.

If you need support or want more information about rare diseases, you might find the websites below helpful.

• Genetic Alliance UK
• EURORDIS – Rare Diseases Europe