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FAQ About Rare Diseases

What is a rare disease?

In the United States, a rare disease is generally considered to be a disease that affects fewer than 200,000 people. Rare diseases are sometimes called orphan diseases.

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How many rare diseases are there?

There are more than 6,800 rare diseases. Altogether, rare diseases affect an estimated 25 million to 30 million Americans.

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What causes rare diseases?

The exact cause for many rare diseases remains unknown. Still, for a significant portion, the problem can be traced to mutations (changes) in a single gene. Such diseases are referred to as rare, genetic diseases. Many of these genetic mutations can be passed on from one generation to the next, explaining why certain rare diseases run in families.

It is important to keep in mind that genetics are just one piece of the puzzle. Environmental factors, such as diet, smoking, or exposure to chemicals, also can play a role in rare diseases. Such factors may directly cause disease, or interact with genetic factors to cause or increase the severity of disease.

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What are some examples of rare diseases?

Examples of rare diseases caused by mutations in single genes include:

  • Cystic fibrosis, which affects the respiratory and digestive systems.
  • Huntington's disease, which affects the brain and nervous system.
  • Muscular dystrophies, which affect the muscles.

Single genes are also responsible for some rare, inherited types of cancer. Examples of these are the BRCA1 and BRCA2 genes, in which certain mutations increase the risk for hereditary breast and ovarian cancers, and the FAP gene, in which mutations increase the risk for hereditary colon cancer.

Rare diseases related to environmental factors include uncommon types of anemia caused by vitamin-deficient diets or certain medications. A rare cancer caused by environmental factors is mesothelioma, which affects the cells lining the chest cavity. More than 90 percent of mesothelioma cases are caused by exposure to asbestos, a fibrous mineral once widely used in fireproofing and insulation materials.

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What is being done to develop treatments for rare diseases?

Researchers have made considerable progress in recent years in figuring out ways to diagnose, treat and even prevent a variety of rare diseases. Still, much more remains to be done because there are no treatments for the vast majority of rare diseases.

The Orphan Drug Act of 1983 provides incentives for drug companies to develop treatments for rare diseases. In the 25 years since the Act was signed into federal law, the U.S. Food and Drug Administration (FDA) has approved more than 340 treatments for rare diseases.

NIH launched the Therapeutics for Rare and Neglected Diseases (TRND) program to create an integrated research pipeline to jump start the development of new treatments for rare and neglected disorders.

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Where can people get more information about rare diseases?

NIH established the Office of Rare Diseases Research (ORDR) to help advance research focused on rare diseases.

To provide patients and their families with timely and reliable information, ORDR and the National Human Genome Research Institute created the searchable Genetic and Rare Diseases (GARD) Information Center. Users can also contact GARD's information specialists directly by email, telephone, fax, TTY or postal mail.

In addition to GARD, there are many nonprofit groups that provide information and support, promote research and advocate for policy issues related to various rare diseases. Among the groups that span a broad range of disorders are the National Organization for Rare Disorders and the Genetic Alliance.

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Learn More About Rare Diseases

The Office of Rare Diseases Research (ORDR) supports and coordinates rare disease research, responds to research opportunities for rare diseases and provides information on rare diseases. Learn more about ORDR.