What is Rare Disease?

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Seoul National University Hospital’s Center for Rare Diseases is the “Central Support Center for Rare Diseases” in Korea, aiming for rapid diagnosis and comfortable return to the base center through a network connection with regional centers.

The Definition of Rare Disease in the Republic of Korea​

Seoul National University Hospital is moving from its Korean roots to become a world-class medical institution.


A rare disease is defined as a condition in which there are less than 20,000 patients with diseases without an estimated number of patients due to difficulties in diagnosis.

Among them, diseases with a population of 200 or less are classified as extremely rare. Rare diseases are either hereditary or congenital diseases, with more than 80% occurring mainly at an early age.

In addition, the diagnosis of rare diseases is difficult due to the small number of patient sizes and specialized medical staff. Health risk persists in rare disease patients, leaving various complications and sequela.

Although the number of diseases varies depending on the classification method, 1,248 diseases are recognized as rare in Korea and medical expenses are covered by national healthcare as of November 2023.

In order to enhance the public’s understanding of rare diseases and promote the prevention, treatment, and management of rare diseases, the Korea Center for Disease Control and Prevention has established May 23rd as the “Rare Disease Overcoming Day” and has been carrying out public relations activities.

Definition of rare diseases in other countries
○ There is no single definition of a rare disease. Some are based on the number of patients while others consider factors such as the severity of the disease and the existence of appropriate treatments.
– The WHO, US, EU, etc. define rare diseases based on the patient population/prevalence rate, while Japan considers uncertainty in the pathogenesis, the existence or lack thereof treatments, and the necessity of long-term care and rest, and Korea defines them based on the prevalence rate and difficulty in diagnosis.

Comparison of rare disease definitions by country

Country / Organization Summary of definition Patient per 100,000 individuals Patient population cap
WHO Rare disease: A disease or condition that affects 0.65 to 1 out of 1000 residents. 6.5 ~ 10 -
US Rare disease and disorders are those which affect small patient populations, typically populations smaller than 200,000 individuals in the US. 6.4 200,000
EU Rare diseases are diseases with a particularly low prevalence; the European Union considers diseases to be rare when they affect not more than 5 per 10,000 persons in the European Union. 5 185,000
Japan Intractable diseases (nanbyou) are defined by uncertainty in the pathogenesis, the existence or lack thereof treatments, and the necessity of long-term care and rest. 3.9 50,000
Taiwan Rare disease: A disease with a low prevalence rate classified and announced as a rare disease by the central authority and the Rare Diseases and Medicine Review Committee. 1 -
Australia Rare disease: Any disease with less than 2,000 patients, regardless of the difficulty of the treatment. 1.1 2,000
Korea Rare disease: A disease with less than 20,000 patients or one that is difficult to diagnose and as such impossible to define the exact number of patients. 4.25 20,000