How Many Rare Diseases Are There?

Updated July 1, 2019

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Rare artwork, sports cards, wine — often rarity is a good thing. Having a rare disease is an exception. Rare diseases affect a small number of people, and they are often medical mysteries.

What's in this Guide?

Disclaimer: Before You Read

It is important to know that your genes are not your destiny. There are various environmental and genetic factors working together to shape you. No matter your genetic makeup, maintain ideal blood pressure and glucose levels, avoid harmful alcohol intake, exercise regularly, get regular sleep. And for goodness sake, don't smoke.

Genetics is a quickly changing topic.

Because they aren’t prevalent, rare diseases may be difficult to identify, diagnose and treat. At, we want you to understand the term and what it means to any diagnosis you or a loved one may have.

What Is a Rare Disease? 

Rare diseases definition: Any disease that affects a very small percentage of the general population 1

In the U.S. a disease is considered rare if it affects less than 200,000 people. In Europe, rare diseases are those that affect less than 1 in 2,000 people. 

Rare Diseases in Children

There are many rare diseases in children that develop either in utero or shortly after birth. It’s estimated that the majority (80%) of rare diseases affect children.

Difference Between Rare and Orphan Disease

An orphan disease is another name for a rare disease. The term was coined decades ago when drug manufacturers choose not to develop treatments for certain rare diseases.

Congress stepped in and created the Orphan Drug Act of 1983 to offer incentives for developing drugs that treat rare diseases. 

Characteristics of Rare Diseases 

No two rare medical conditions are exactly alike. They each have their own characteristics that set them apart and make diagnosis possible. 

The only characteristic that is shared is the limited number of people who are affected. 

Rare Diseases List and Symptoms

There are a number of organizations that maintain lists of rare diseases. Orphanet has a list of rare diseases by prevalence and information on the number of published cases 2. The Genetic and Rare Diseases Information Center also has a list of rare conditions with symptoms for each. 

How Many Rare Diseases Are There? 

Currently, 7,000 rare diseases have been identified. Due to the nature of rare diseases and potential isolation within a certain population, there may be other rare diseases that haven’t been discovered yet.

The Most Rare Diseases in The World 

Some rare diseases are less common than others. There are hundreds of diseases that only a few dozen or fewer people have. The most rare diseases in the world include: Hutchinson-Gilford progeria, N-Glycanase deficiency, Gaucher disease and hereditary angioedema.

Some of these are extremely rare genetic disorders that affect a select number of people that share familial bonds. 

What Percentage of People Have a Rare Disease? 

Rare disease statistics find that there are likely between 25-30 million people in the U.S. who have a rare disease. That would mean about 7.5-9% of the American population has a rare disease. 

When you look at it that way, rare diseases don’t seem so rare. However, you have to keep in mind those numbers are spread out over approximately 7,000 diseases. That means on average a rare disease affects just 3,500 to 4,300 people.

One factor that can skew the numbers is rare disease tracking. Many rare diseases aren’t tracked, which makes it hard to know for sure how many people have a disease.

While all rare diseases aren’t prevalent in the population, some affect many more people than others. Some of the most common rare diseases are:

  • Cystic fibrosis
  • Multiple sclerosis 
  • Duchenne muscular dystrophy
  • Amyotrophic lateral sclerosis (ALS)
  • Acute lymphocytic leukemia
  • Lou Gehrig’s disease
  • Muscular dystrophy
  • Measles
  • Sickle-cell disease
  • Alpha-1 antitrypsin deficiency

The Origins and Causes of Rare Diseases 

Rare diseases can originate at various stages of life depending on the cause. Below are some of the known causes of rare diseases.

Genetic Mutations

Often, rare diseases are genetic, which is why they affect only a select number of people. It is believed that about 80% of rare diseases have a genetic component.

The genetic changes that cause the disease can happen randomly or they may be passed down from parents to children. 

Birth Defects

A number of rare diseases develop in utero and are known as birth defects. Birth defects can occur for a number of reasons and sometimes the cause is unknown. In many cases there is a genetic link or the fetus was exposed to a toxin.


Cellular mutation leads to cancer, some forms of which fall into the rare disease category. 


Exposure to a virus or bacteria is another common cause of rare diseases. 

Unfortunately, the causes of many rare diseases remain unknown. As research progresses doctors hope to have answers that lead to better treatment options and prevention of rare diseases. 

Common Problems Faced With Rare Diseases 

As mentioned earlier, treatments and therapies for rare diseases may not be well researched or developed. Drug manufacturers typically focus their efforts on more common diseases that affect a larger population. 

Diagnosing a rare disease may also be a problem. Doctors usually look for and rule out more common medical conditions before exploring the possibility of a rare one.

The diagnosing physician may not be familiar with the rare disease and therefore could misdiagnose or fail to discover the problem. It’s not uncommon for a rare disease to go undiagnosed until a person is able to see a specialist that knows the signs and symptoms. 

Diagnosis of Rare Diseases 

Diagnosing a rare disease is quite often a difficult task if there is no known family history because very little information exists. Much of the time the symptoms seem unrelated or there isn’t an obvious connection to the rare disease 3.

It’s such a challenge diagnosis can take years, which is referred to as a diagnostic odyssey. 

After ruling out common ailments with similar symptoms, doctors sometimes hit a roadblock. Many have never had firsthand experience treating rare diseases.

They may also mistakenly misdiagnose a disease only to retract the diagnosis after more symptoms manifest or don’t respond to treatment. 

Some patients have to visit many doctors and specialists before getting an answer. Even after diagnosing a rare disease a new set of issues can arise if there is no clear treatment available.

People with rare diseases often have to travel great distances to get to specialists who are experienced enough to make a diagnosis. The path to figuring out what’s wrong — and if there's a treatment that can help — can take an emotional and financial toll on patients and loved ones.

Managing and Treatment of Rare Diseases 

Managing and treating a rare disease often requires the expertise of a rare disease specialist. Even with the help of a specialist, some diseases are difficult to manage because there are no treatments. 

In the past 30 years, hundreds of treatments for rare diseases have been developed thanks to gene mapping and the Office of Orphan Products Development (OOPD). Despite that, less than 500 rare diseases have FDA-approved treatments 4.

Rare diseases that can be cured or treated include:

  • Non-Hodgkin lymphoma
  • Melorheostosis
  • Hereditary angioedema
  • Fabry disease
  • Beta thalassemia
  • Amyotrophic lateral sclerosis (ALS)
  • Juvenile idiopathic arthritis
  • Acute lymphocytic leukemia
  • Chronic lymphocytic leukemia
  • Cystic fibrosis
  • Multiple myeloma
  • Pulmonary arterial hypertension
  • Hypophosphatasia (HPP) 
  • Lysosomal acid lipase (LAL) deficiency
  • Neuroblastoma 

At the rate medicine is moving, it’s difficult to say how many rare diseases have treatments right around the corner or in the near future.

Progress in Fighting Rare Diseases 5

Like many other industries, medicine has become globalized. Doctors and researchers around the world can more easily share information and their findings in an effort to expedite rare disease research. 

The National Center for Advancing Translational Sciences (NCATS) is an example of this. NCATS sponsors collaborative research in an effort to find causes and treatments more quickly. 

The study of rare diseases has grown in the last few decades largely because of a better understanding of genetics. Unlocking the genetic code has allowed researchers to pinpoint the cause of numerous rare diseases. 

In some cases, doctors are able to determine if DNA passed down from a single parent is enough to cause a disease or if DNA must come from both parents.

Genetics research has also been able to determine if spontaneous genetic mutations are the cause of some rare diseases and therefore the affected individual doesn’t have to worry about passing the disease on to a child.

Much of the research is conducted through the National Human Genome Research Institute (NHGRI).

The silver lining is that many top researchers aren’t afraid to take on diseases and disorders that are rare. Organizations such as the National Institutes of Health (NIH) are also providing support and funding for research into rare conditions. 

The government is continuing to provide incentives for rare disease research through the Office of Orphan Products Development (OOPD). The OOPD program was launched in 1983 and since that time the development of treatments for rare diseases has grown from 10 treatments to over 770 approved therapies.

Hundreds of medicines are also in development. Of those, over 150 medicines are aimed at treating cancers and nearly 150 are for genetic disorders.

Rare fatal childhood diseases get a lot of focus even when a small number of children are affected. Since the majority of rare diseases are inherited, children are more likely to be affected by these types of diseases than adults.

Fortunately, doctors have made major headway treating some of the top 10 rare childhood diseases like acute lymphocytic leukemia.

The truth is, studying rare diseases can lead to treatments for common diseases as well.

One of the best known examples of this happened in the 1960s when researchers were studying the cause of a rare disease called familial hypercholesterolemia, which is marked by extremely high cholesterol levels at a young age.

The research led to the development of statins, a drug that is now used to lower cholesterol and treat heart disease in millions of people. 

Where Can I Get More Information About Rare Diseases? 

"I have a rare disease and would like to find out more about my condition. I’d also like to make connections with people who have also been affected by the rare disease." 

If those thoughts have ever crossed your mind, know that there is help and support for people with just about every type of rare disease. Some organizations include:

  • The National Organization for Rare Disorders (NORD) is an excellent resource for learning more about a rare disease and the latest research. NORD maintains a database with brief descriptions of rare diseases 
  • Genetic and Rare Diseases Information Center (GARD) is highly beneficial for those who have rare diseases, especially ones with a link to a genetic cause. GARD has a comprehensive list of diseases that are believed to be rare along with information on which conditions are most likely to be genetic or increase the likelihood of a rare disease. Information is updated regularly with the latest findings and discoveries. You can also learn about ongoing research for a rare disease. Another highly beneficial feature is that GARD information specialists are available to answer questions over the phone.
  • You can find statistics and facts about a specific rare disease in the Medscape Reference, Genetics Home Reference and GeneReviews. 
  • The National Library of Medicine is a great source for information and medical studies involving rare diseases. 
  • The Rare Disease Network, also known as the Rare Diseases Clinical Research Network (RDCRN), supports clinical studies for rare diseases and data sharing. Currently, the RDCRN helps facilitate research for 200+ rare diseases. 
  • The Scientific Society for Rare Diseases & Orphan Drugs (S.S.R.D.O.D.) is a network for doctors and researchers that are working towards finding treatments for rare diseases.

Rare Disease Patient Support Groups

In our internet age, people are able to connect no matter where they live. This has given patient support groups a platform for even the rarest diseases.

Now, people living with these conditions can become a part of rare disease communities and form relationships with others that understand what they are going through. 

There are many patient support groups that can be found online and even on social media platforms like Facebook. The Genetic and Rare Diseases Information Center (GARD) is an excellent resource for finding support groups. As is the Rare Disease United Foundation.

The importance of emotional support cannot be overstated. Knowing they are not alone gives people hope and serves as a way to share concerns about rare diseases when other types of assistance are limited.

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Referenced Sources

  1. FAQs About Rare Diseases.
    National Center for Advancing Translational Sciences, 11/30/2017.
  2. Prevalence and incidence of rare diseases: Bibliographic data.
    Ana Rath, Stephanie Nguengang Wakap, Samuel Demarest, Valerie Lanneau, 2014.
  3. Profile of Rare Diseases.
    National Academy of Sciences, 2010.
  4. Regulatory Framework for Drugs for Rare Diseases.
    MJ Field, TF Boat, National Academies Press. 2010.
  5. Progress in Fighting Rare Diseases.
    PhRMA, 2019.