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What is a rare disease?

Illustration showing people affected by rare diseases

29 February 2024 marks Rare Disease Day. But what exactly is a rare disease, and how widespread are they?

Definitions of rare diseases can vary depending on where you live. The US National Institutes of Health (NIH) defines a disease as rare if it affects fewer than 200,000 Americans. The European Union describes a rare disease as one that affects fewer than 1 in 2,000 people. 

There are between 6,000 and 8,000 known rare diseases, so while the conditions themselves are rare, being affected by one is quite common. Around 300 million people worldwide 1 are affected by a rare disease, and it can take up to 5 years to receive a correct diagnosis. 

When does rare disease onset begin?

Rare diseases can present with a wide range of symptoms that can at first seem unrelated. It's estimated that around 71.9% of rare diseases have a genetic link. The onset of around 69.9% of rare diseases begins in childhood. 11.9% begin exclusively in adulthood, and the onset of 18.2% of rare diseases occurs in both children and adults. 2

What are some well-known rare diseases?

Some rare diseases are more widely recognized than others. You may have heard of conditions like hemophilia (which affects the blood’s ability to clot), Ehlers-Danlos syndrome (a group of conditions affecting connective tissues), or cystic fibrosis (which affects the cells that produce mucus, sweat, and digestive juices). 

Some rare diseases are immediately visible to other people, whereas others do not present with any outwardly noticeable symptoms or effects. Always take other people at their word, even if they don’t appear to have any physical signs of a condition. 

Treating rare diseases

Rare diseases are uncommon and, therefore, usually harder to diagnose. This also means that there's less research and often fewer treatments for these conditions. 

Rare disease campaigners estimate that 95% of rare diseases do not have specific licensed treatments. 3 This means that many symptoms of these conditions are treated with medications that were developed for other, more common conditions. Encouraging the development of adequate treatments is one of many reasons why growing awareness of rare diseases is key. 

Key issues facing rare disease patients and carers

Rare disease awareness is necessary to combat the many obstacles facing rare disease patients and their carers. 

  • The average rare disease patient can wait up to 5 years for an accurate diagnosis
  • On average, they will consult with 5 doctors and receive 3 misdiagnoses before they receive the correct one.
  • One-third of rare disease patients do not have access to the medication they need.
  • One-third of rare disease patients will wait years before accessing any kind of treatment.
  • Treatments that do exist for rare diseases can cost up to 7 times more than treatments developed for more common conditions.
  • 2 in 3 patients and carers struggle to hold onto paid employment. 4

Getting a diagnosis for a rare disease is just the first step to living with the condition long-term. By raising awareness and engaging with content that educates you about the realities of living with a rare disease, you are part of a community fighting for better treatment and recognition of rare conditions.

If you think you might have a rare disease, why not check your symptoms with the Ada app? We’ve modeled hundreds of rare diseases, and our app can check your symptoms against thousands of conditions and billions of possible symptom combinations. It’s our mission to empower patients and reduce the time to diagnosis for rare conditions. We look forward to continuing this work.

Take care of yourself,


  1. Rare Disease Day (2023) What is a rare disease? Accessed on 17 February 2024.

  2. European Journal of Human Genetics (2019) Estimating cumulative point prevalence of rare diseases: analysis of the Orphanet database. Accessed on 17 February 2024.

  3. Beacon for rare diseases (2024) What are rare diseases? Accessed on 17 February 2024.

  4. Rare Disease UK (2019) Illuminating the Rare Reality. Accessed on 17 February 2024.



Ada is a global health company founded by doctors, scientists, and industry pioneers to create new possibilities for personal health.

Medical reviewer:


Ada is a global health company founded by doctors, scientists, and industry pioneers to create new possibilities for personal health.