How Rare are Rare Diseases?

Why it matters?

Rare diseases are not common as individual syndromes in themselves. Some of them affect only a handful of people worldwide. Others, such as Down syndrome, are more prevalent and affect thousands of people. Generally, in the US, a rare disease is one which affects less than 200,000 people. 

However with 7000 currently diagnosed, and more being identified and named all the time, there are currently anywhere between 25 and 30 million Americans with a genetic syndrome. This doesn’t account for those who go undiagnosed, due to mild symptoms, wrong diagnosis, and just a lack of awareness about what a genetic disease might look like. 

They may be uncommon, as patients of individual conditions, but collectively rare disease patients are not a small amount of people. 

Often they are also called orphan diseases, or a zebra disease. This is because there is a lack of research into their causes and symptoms, and not much is generally understood about them. This can make them difficult to diagnose, and leave many patients stranded in what is known as a diagnostic odyssey. 

But the more we understand about them, the more accurately we can diagnose them, and the more we understand about rare disease causes, the more we can empower individuals and families when it comes to their genetic health. 

Greater rare disease awareness means ensuring that patients get the support and possible treatment they need, faster. 


Many rare diseases are prevalent in less than 1 in 1 million people. Examples of these include Kleefstra syndrome, LEOPARD syndrome, and Witteveen-Kolk syndrome

Sometimes the exact prevalence is unknown, as many people are undiagnosed or wrongly diagnosed, a common issue when it comes to genetic diagnosis. Ehlers Danlos syndrome, and the 13 known types of it, is one example of a condition that is commonly misdiagnosed, and for which it is believed there are possibly large numbers of undiagnosed affected individuals. 

For some diseases, even an approximate estimation of their prevalence is as yet unknown. Syndromes with currently unknown prevalence include Warburg Micro syndrome, Urofacial syndrome, Temple syndrome, and Adams-Oliver syndrome

Some are more prevalent in specific communities, or amongst specific ethnicities. This includes Tay-Sachs, which is more common in those with European Ashkenazi Jewish ancestry. 

The figures

We break it down below –

Rare Disease Approximate prevalence
Turner syndrome 1 in 2,000
22q11.2 deletion syndrome 1 in 4-7,000
Angelman syndrome 1- 12-20,000
Joubert syndrome 1 in 20,000
Kabuki syndrome 1 in 30-80,000
Basal Cell Nevus syndrome 1 in 60,000
Silver-Russell syndrome 1 in 100,000

There is still much to be learnt about rare diseases – both in terms of their causes and symptoms, and the more we increase this knowledge and understanding, the more we empower patients with an accurate genetic diagnosis, and the right support.

FDNA™ Health can bring you closer to a diagnosis.
Learn here about symptoms and conditions and contact your clinician.