A disease that affects less than 5 in 10,000 of the general population (EU definition). In the UK, approximately 3.5 million people will be affected by a rare disease at some point in their life (Rare Disease UK).
Use in clinical context
Individually a rare disease is rare, but collectively they are common. Many people with rare diseases often go through a diagnostic odyssey with many tests that do not necessarily lead to an explanation or treatment for their disease. It is thought that the majority of rare diseases have a genetic cause. Finding this cause can lead to a diagnosis and possibly treatment options. Increasingly, more people with undiagnosed rare diseases are undergoing genomic testing which is providing a diagnosis for some. Not everyone who has a genomic test will receive a diagnosis initially, but those without a diagnosis may receive one as more is discovered about the function of the genome.