Rare diseases cover a huge range of different, specific diseases and conditions. We’ve pulled together some of the most common questions on rare diseases and orphan drugs to give you a better understanding of this important topic. If you want to read even more information, try our comprehensive blog: rare diseases: everything you need to know.
Simbec-Orion is a specialist rare disease CRO, supporting pharmaceutical companies and other organisations in researching orphan drug development.
We have split this blog into sections to make it easier to navigate to the area you’re most interested in learning more about:
What is a rare disease?
A rare disease is a condition that affects fewer than 1 in 2,000 people (UK / EU definition). The majority of rare diseases don’t have an effective treatment.
This definition can vary in different places around the world. In the US, a rare disease is classed as a condition that affects fewer than 200,000 Americans, as defined by the Orphan Drug Act.
Rare diseases most commonly have a genetic origin – about 72-80% of rare conditions are genetic, and a large proportion of genetic conditions are rare (source). However, many are caused by other triggers such as immune disorders, infections, allergies, disrupted foetal development, and more.
Some diseases might be considered rare, but are more prevalent in certain age, sex or racial groups.
How many people have a rare disease?
While each rare disease itself is uncommon, it’s estimated that 1 in 17 people in the UK are affected by a rare disease during their lifetime – that’s over 3.5 million people. (Source)
In the United States, it’s estimated that around 1 in 10 people have a rare disease (source) – more than 30 million people.
Worldwide, evidence-based estimates suggest rare diseases affect 3.5–5.9% of the global population (263–446 million people). However, when measuring the impact of rare diseases through family members and carers, RD affects approximately 1.05-1.4 billion people globally.
Rare diseases are more common in children than in adults. About half of the people in the world who have a rare disease are children, and of the ~7,500 known rare diseases, about 75% affect children (source). According to a study by Wakap et al, almost 70% of rare diseases have an exclusively paediatric (childhood) onset.
What is considered an orphan disease?
Orphan diseases are another name for rare diseases. They affect fewer than 1 in 2000 people (European definition, Fewer than 200,000 in the US). It’s often used to denote conditions where there is not a large enough affected population to gain support to fund research. This often means that there are fewer treatments for those with rare diseases.
What is considered a very rare disease?
A ‘very rare’ disease is not a medically recognised term. If a disease or condition affected just 1 person, it would still be classed as a ‘rare disease’. Some conditions are described as ‘ultra-rare’, typically affecting fewer than 1 in 50,000 people. Although not a formal legal classification, it is a widely used term in rare-disease research and by organisations
How many rare diseases are there?
Experts now agree there are “more than 10,000” rare diseases and counting as genomic research identifies new conditions. As research progresses, new diseases are being discovered. Globally, exact definitions of what a rare disease is vary, impacting the number of diseases classed as rare by different sources.
What are some examples of rare diseases?
There are more than 10,000 known rare diseases, and some are more well-known than others. These are just a small list of rare diseases:
- Cystic fibrosis
- Spina bifida
- Haemophilia
- Sickle Cell
- Mycosis fungoides (a form of Lymphoma)
- Ehlers-Danlos Syndrome (EDS)
There are databases available that have lists of rare diseases:
- Orphanet Database for Rare Diseases and Orphan Drugs
- NORD Rare Disease Database
- Global Genes RARE list
Top Ten Rarest Genetic Disorders
While it’s impossible to create a definitive ranking, the following conditions are among the rarest known genetic disorders worldwide, many with only a handful of documented cases or affecting just a few individuals per million people.
- Ribose-5-phosphate isomerase (RPI) deficiency – fewer than 10 known cases worldwide
- Fibrodysplasia ossificans progressiva (FOP) – occurs in approximately 1 in 1 million people worldwide
- Hutchinson-Gilford progeria syndrome (HGPS) – Affects 1 in 4 million newborns worldwide
- NGLY1 deficiency – Approximately 100 individuals are affected
- Methemoglobinemia type II – about 1 in 1 million
- Xeroderma pigmentosum (XP) – Affects 1 in 100,000 individuals worldwide.
- Alkaptonuria – Fewer than 5,000 people in the U.S.
- Microcephalic osteodysplastic primordial dwarfism type II – Rare with unknown prevalence
- Paraneoplastic pemphigus (PNP) – Rare with unknown prevalence
- Fields’ disease – only two known cases worldwide
Treating rare diseases
What type of drug is used to treat a rare disease?
There is no single drug used to treat a rare disease because rare diseases have a wide range of causes and effects. For example, they can include rare cancers through to neurological conditions.
Many rare diseases don’t have viable treatment options due to limited research and funding. Recent reviews estimate that ~95% of rare diseases lack approved treatments.
What is an orphan drug in clinical trials?
Orphan drugs in clinical trials are any drugs that are being tested to treat rare diseases that have ‘orphan’ status.
What does orphan status of a drug mean?
If a drug is classed as an ‘orphan drug’, it means it’s being used or investigated for use in treating a rare disease. Orphan drug designation means sponsors qualify for research incentives such as tax deductions. This is to help rare diseases get more research and funding.
The population number affected by the disease in order for it to be classed as rare is different in different countries.
In the EU, the European Medicines Agency also defines an orphan drug as a drug that, without other incentives, is unlikely to be used by or benefit enough people to meet the manufacturer’s financial investment.
Without incentives, the treatments for many rare diseases would never be found because it would not be financially viable to research them. The goal of giving a drug orphan status, therefore, is to create incentives for sponsors and manufacturers, so new treatments for rare diseases can be found.
What is the most popular orphan drug?
Recent market analyses (Evaluate, 2024–2025) indicate that Daratumumab (Darzalex) and Elexacaftor/tezacaftor/ivacaftor (Trikafta) rank among the top-selling orphan medicines, with Darzalex projected to remain #1 by 2028–2030. Rankings vary by year and methodology (Sources).
How many treatments are there for rare diseases?
There are many rare disease treatments and orphan drugs, and dozens are approved globally each year. However, 95% of rare diseases still have no treatment.
What are some examples of drugs for rare diseases?
Examples of orphan drugs for rare diseases include:
| Drug | Brand name | Used to treat |
| Tafamidis | Vyndaqel | Transthyretin amyloidosis |
| Acalabrutinib | Calquence | Non-Hodgkin lymphoma |
| Ibrutinib | Imbruvica | Certain blood cancers |
| Ravulizumab | Ultomiris | Paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome |
| Elexacaftor/tezacaftor/ivacaftor | Trikafta | Cystic fibrosis |
Rare disease day questions
What is Rare Disease Day?
Rare Disease Day is a global event to help raise awareness of rare diseases and encourage organisations to improve access to treatment for those affected. It was established by the European Organisation for Rare Diseases in 2008 and is now observed by people all over the globe.
When is Rare Disease Day?
28th February (or 29 February in leap years – the “rarest” day). It’s on the last day of February every year.
What is the theme for Rare Disease Day 2026?
The official theme for 2026 has not yet been announced on the campaign website, but historically, the event has consistently used the “Share Your Colours / Light Up for Rare” motif and, for 2025, adopted the message “More Than You Can Imagine.” (source: RareDiseaseDay.org)
Support for rare diseases
How can we help people with rare diseases?
If you know someone with a rare disease, the best way to support them is to ask them what they need. Every person will be different: some people might want a friendly ear to listen to how they’re feeling; others might need practical help, such as delivering a meal or providing transport; others might just appreciate knowing that you’re available, even if they don’t need help right now.
What resources are there to support people affected?
There are lots of resources available for families and friends of people with rare diseases, as well as people with rare diseases themselves.
https://www.orpha.net/en/disease
https://geneticalliance.org.uk/campaigns-and-research/rare-disease-uk
https://www.rareconnect.org/en
Facebook/social media support groups
Can you participate in a clinical trial?
Sometimes, there might be orphan drug or rare disease clinical trials being conducted. Patient recruitment for rare disease trials can be difficult as there aren’t as many people affected by the disease, so if you want to help further research into treatment options and feel comfortable participating, it can be a productive way to help.
Clinical trials often require both healthy volunteers in the early stages and then people who are affected by the target condition.
This can be an effective way to improve the treatments available for certain rare diseases.
Rare Disease Clinical Trials at Simbec-Orion
If you have more questions about clinical trials for rare diseases, the Simbec-Orion team are specialists, having 50 years of experience in the management of phase 1, phase 2 and phase 3 . Get in touch for more information,