Rare diseases affect small groups of patients and are often life-threatening or cause chronic disability. In total there are about 7,000 known rare diseases but only about 400 approved treatments for this group of conditions.
Examples of serious or life-threatening rare illnesses include haemophilia, malignant melanoma, essential thrombocythaemia, soft tissue sarcoma, ovarian cancer and metabolic disorders such as hereditary tyrosinaemia type I and insufficiency of the urea cycle.
In the EU a rare disease is defined as one that affects less than 1 of 2,000 people (or 0.05% of the population). In the US, rare diseases are defined as those that affect less than 200,000 people.