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A rare disease is any disease that is known to affect fewer than 200,000 people in the US. This is the definition used by the US Food and Drug Administration (FDA) to determine whether a treatment may be eligible for "orphan" drug status.
According to the National Institute of Health (NIH), there are approximately 6,000 rare diseases affecting more than 25 million Americans. Even though each disease may affect only a small number of people, rare diseases collectively have a significant impact.
A genetic disease is a disease caused by a different form of a gene called a variation, or an alteration of a gene called a mutation. Many diseases have a genetic aspect. Some, including many cancers, are caused by a mutation in a gene or group of genes in a person's cells. These mutations can occur randomly or because of an environmental exposure such as cigarette smoke. Other genetic disorders are inherited. A mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease. Still other genetic disorders are due to problems with the number of packages of genes called chromosomes.