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A rare disease, sometimes known as an orphan disease, is any disease that is not common. Typically, a rare disease has such a low prevalence in a population that a physician in a busy general practice would not expect to see more than one case a year.

Most rare diseases are genetic, and thus are present throughout the person's entire life, even if symptoms do not appear immediately. However, many rare diseases appear early in life, and about 30% of children with rare diseases die before reaching their fifth birthdays.

No single cutoff number has been agreed upon for which a disease is considered rare. A disease may be considered rare in one part of the world, or in a particular group of people, but still be common in another.

No single definition

There is no single, widely accepted definition for rare diseases. Some definitions rely solely on the number of people living with a disease, and other definitions include other factors, such as the existence of adequate treatments or the severity of the disease.

In the United States of Americamarker, the Rare Disease Act of 2002 defines rare disease strictly according to prevalence, as "any disease or condition that affects less than 200,000 persons in the United States," or about 1 in 1,500 people.

In Japan, the legal definition of a rare disease is one that affects fewer than 50,000 patients in Japan, or about 1 in 2,500 people.

However, the European Commissionmarker on Public Health defines rare diseases as "life-threatening or chronically debilitating diseases which are of such low prevalence that special combined efforts are needed to address them." The term low prevalence is later defined as generally meaning less than 1 in 2,000 people. Diseases that are statistically rare, but not also life-threatening, chronically debilitating, or adequately treated, are excluded from their definition.

The definitions used in the medical literature and by national health plans are similarly divided, with definitions ranging from 1/1,000 to 1/200,000.

Relationship to orphan diseases

Because of definitions that include reference to treatment availability, a lack of resources, and severity of the disease, some people prefer the term orphan disease and use it as a synonym for rare disease. The orphan drug movement was begun in the United States.

Others distinguish between the two terms. For example, the European Organization for Rare Diseases (EURORDIS) lumps both rare diseases and neglected diseases into a larger category of orphan diseases.

The United States' Orphan Drug Act includes both rare diseases and any non-rare diseases "for which there is no reasonable expectation that the cost of developing and making available in the United States a drug for such disease or condition will recovered from sales in the United States of such drug" as orphan diseases.


Prevalence (number of people living with a disease at a given moment), rather than incidence (number of new diagnoses in a given year), is used to describe the impact of rare diseases.

The European Organization for Rare Diseases (EURORDIS) estimates that there exist between 5,000 and 7,000 distinct rare diseases. Although each individual disease is rare, the sheer number of individual rare diseases results in between 6% and 8% of the population of the European Union being affected by a rare disease.

Rare diseases can vary in prevalence between populations, so a disease that is rare in some populations may be common in others. This is especially true of genetic diseases and infectious diseases. An example is cystic fibrosis, a genetic disease: it is rare in most parts of Asia but relatively common in Europe and in populations of European descent. In smaller communities, the founder effect can result in a disease that is very rare worldwide being prevalent within the smaller community. Many infectious diseases are prevalent in a given geographic area but rare everywhere else. Other diseases, such as many rare forms of cancer, have no apparent pattern of distribution but are simply rare. The classification of other conditions depends in part on the population being studied: All forms of cancer in children are generally considered rare, because so few children develop cancer, but the same cancer in adults may be more common.

About 40 rare diseases have a far higher prevalence in Finlandmarker; these are known collectively as the Finnish disease heritage.


Rare diseases usually are genetic, hence chronic. EURORDIS estimates that at least 80% of them have identified genetic origins. Other rare diseases are the result of infections and allergies or due to degenerative and proliferative causes.

Classification of a disease's rarity also depends on the population being studied. Every form of cancer is rare among children, but some forms are common among adults.

Symptoms of some rare diseases may appear at birth or in childhood, whereas others only appear once adulthood is reached.

Research publications emphasize rare diseases that are chronic or incurable, although many short-term medical conditions are also rare diseases.


The National Organization for Rare Disorders was established in 1983 by individuals and families with rare diseases. The NIH's Office of Rare Diseases was established by H.R. 4013 in 2002. H.R. 4014, signed the same day, refers to the "Rare Diseases Orphan Product Development Act". Similar initiatives have been proposed in Europe. Genetic Alliance lists support groups for approximately 1000 rare diseases.

The first Rare Disease Day was held in Europe and Canadamarker in February 2008 to raise awareness for rare diseases. It is intended to be observed on the last day of February every year.

See also


  2. Rare Disease Act of 2002]
  3. Rare diseases: what are we talking about?
  4. Orphan Drug Act ยง526(a)(2)

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