A rare disease, also referred to as an orphan disease, is any disease that affects a small percentage of the population.
Most rare diseases are genetic, and thus are present throughout the person's entire life, even if symptoms do not immediately appear. Many rare diseases appear early in life, and about 30 percent of children with rare diseases will die before reaching their fifth birthday. With a single diagnosed patient only, ribose-5-phosphate isomerase deficiency is presently considered the rarest genetic disease.
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.
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, the Rare Diseases Act of 2002 defines rare disease strictly according to prevalence, specifically "any disease or condition that affects less than 200,000 persons in the United States," or about 1 in 1,500 people. This definition is essentially like that of the Orphan Drug Act of 1983, a federal law that was written to encourage research into rare diseases and possible cures.
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 Commission 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 fewer than 1 in 2,000 people. Diseases that are statistically rare, but not also life-threatening, chronically debilitating, or inadequately 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[who?] prefer the term orphan disease and use it as a synonym for rare disease. The orphan drug movement began 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 [be] 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 Global Genes Project estimates there are some 350 million people worldwide currently affected with a rare disease.
The European Organization for Rare Diseases (EURORDIS) estimates that as many as 5,000 to 7,000 distinct rare diseases exist, and as much as 6% to 8% of the population of the European Union is affected by one.
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.
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.
Symptoms of some rare diseases may appear at birth or in childhood, whereas others only appear once adulthood is reached.
Public research 
The NIH's Office of Rare Diseases Research (ORDR) was established by H.R. 4013/Public Law 107-280 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.
Public awareness 
The Global Genes Project is one of the leading rare and genetic disease patient advocacy organizations in the world. The non-profit organization is led by Team R.A.R.E. (R.A.R.E stands for Rare disease, Advocacy, Research and Education). Global Genes promotes the needs of the rare and genetic disease community under a unifying symbol of hope – the Blue Denim Genes Ribbon™. What began as a grassroots movement in 2009 with a few rare disease parent advocates and foundations has grown to over 500 global organizations. Global Genes uses a simple concept of "genes and jeans" to broadly promote the needs of the rare and genetic disease community. The organization has launched a number of innovative rare and genetic disease awareness campaigns including, Hope, It's In Our Genes™, Wear That You Care™, 7,000 Bracelets for Hope™ to represent the 7,000 different rare diseases and Unite 1 Million For RARE™ disease.
The Canadian Organization for Rare Disorders (CORD) is the national network of organizations who represent people affected by rare disorders within Canada. CORD's intention is to provide a strong common voice advocating for a healthcare system and health policy for those with rare disorders.
Patients with rare diseases in Greece are represented by the Greek Alliance of Rare Diseases.
See also 
- Global Genes Project
- Genetic Alliance UK
- Orphan drugs
- Rare Disease Day
- Association of Genetic Support of Australasia (AGSA)
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- Orphan Drug Act §526(a)(2)
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- Global Genes Project, Rare and genetic disease patient advocacy organization
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- Database of rare diseases at Orphanet
- Information from the European Organization for Rare Diseases (EURORDIS)
- Health-EU Portal Rare Diseases in the EU
- Orphan Diseasome Rare/Orphan Disease Networks
- NIH Rare Diseases Clinical Research Network (RDCRN)
- OrphanAnesthesia - Anesthesia recommendations for patients suffering from rare diseases
- ICORD - International Conferences on Rare Diseases and Orphan Drugs