A rare disease is any disease that affects a small percentage of the population. There is no single, unanimously accepted definition of the term. In the United States, a rare disease is classified as any disease that affects less than 200,000 Americans in total, whereas in Europe it is defined as diseases or disorders that affect less than 1 in 2000 people.

“Orphan diseases” are sometimes used as a synonym for rare diseases to stress their neglected conditions since the treatments are often not considered profitable due to their high development costs and limited patient population.

There are more than 6000 to 7000 different rare diseases.

While individual diseases may be rare, the total accumulation currently affect 3.5% - 5.9% of the worldwide population, an estimated 5 million people in Turkey, 30 million in Europe and 300 million worldwide.

More than 70% of rare diseases are genetic in origin while remaining are the result of infectious agents (bacterial or viral), toxic substances, adverse effects of treatments, allergies et cetera. One gene or multiple genes may contribute to manifestations of the disorder. They can be inherited or arise in sporadic or chance mutations.

70% of the genetic rare diseases appear early in life, and about 30% of children with rare diseases die before reaching their fifth birthday. The symptoms and etiology are complex and often not well understood, causing many patients to encounter greater challenges in being properly diagnosed or having access to effective treatments.

Only about 400 rare diseases have therapies currently. Nevertheless, researchers have made progress in learning how to diagnose, treat, and even prevent a variety of rare diseases in recent years. In 2018, an estimated 3,500 molecules targeting rare diseases worldwide were active in the R&D process. This number is more than twice the level that existed 10 years ago. In addition, the total number of orphan drug approvals increased from 84 between 1994-98, to 316 in the 4-year period from 2014 to 2018. As the awareness of rare diseases in the world rises in the eyes of researchers, pharmaceutical companies, public administrators and the society, the access of patients to a healthy and participatory life will increase.