How common are rare diseases?

Defining what constitutes a rare disease can vary significantly. In the United States (US), rare diseases are defined as conditions that affect fewer than 200,000 people [1]Jump to reference section: [1], although in the United Kingdom (UK) it’s considered to be conditions that affect less than 1 in 2,000 people [2]Jump to reference section: [2]. Although rare diseases are individually rare, collectively they are common as there are approximately 7,000 rare diseases that have been discovered. Rare diseases affect about 1 in 17 people in the UK; that’s 3.5 million people in the UK and 30 million people across Europe [3]Jump to reference section: [3]. 

Are rare diseases inherited or random?

Most rare diseases (72%) are caused by a genetic change, however some rare diseases are caused by infections, allergies or other environmental factors (rarediseases.org). Genetic rare diseases can be both inherited and randomly occur. Inherited forms of rare diseases are often passed down in an autosomal recessive pattern where both biological parents of the person with a rare disease are carriers for that condition (learn more about carriers in our previous blog post). Inherited forms of rare diseases can also be passed down in an autosomal dominant pattern where one biological parent has a genetic change and passes it down to their child. Genetic rare diseases can also occur randomly, where a brand new genetic change (called a de novo mutation) occurs in an egg, sperm or embryo.