feel isolated and alone as they face the challenges of finding helpful information, learning a new medical language, and generally charting their way in a daunting new world.

As described in Chapter 1, some rare conditions are extremely rare, found in only a few or a few dozen people. Others occur in hundreds, thousands, or as many as 200,000 people in the United States. Many are genetic in origin or have a genetic component. Others arise from exposure to infections or toxins, from faulty immune responses, or occasionally from adverse responses to therapeutic interventions for other conditions. For many rare conditions, the causes are frustratingly elusive.

Although people may think of a rare disease as something that happens to someone else, rare diseases can afflict anyone, at any age. They can be acute or chronic. Many are debilitating and present an ongoing risk of death. Some are inevitably fatal given current medical options. Approved therapies are available to treat several hundred of these conditions, but most currently have no therapy that cures or modifies the disease itself.

For the rarest conditions, the literature may consist of a single published report describing a few individuals with a previously unidentified genetic syndrome. For other conditions, including a number of the relatively more common conditions such as cystic fibrosis, sickle cell disease, and some cancers, publicly and privately sponsored research has generated a knowledge base that may encompass epidemiology (including natural history studies), genetics, disease mechanisms, diagnostic tests and standards, biomarkers and outcome measures, effective treatments, and evidence-based guidelines for clinical services.

Faced with these realities, many patients and families turn to advocacy groups concerned with specific diseases or to umbrella organizations such as the National Organization for Rare Disorders (NORD) and the Genetic Alliance for support and for information about their condition and available resources. As discussed at the end of this chapter, they may also join together to create new organizations.

This chapter begins with a general overview of what is known about the epidemiology of rare diseases based on data and analyses from the United States and Europe. Epidemiologic studies can provide clues and directions for basic and clinical research to determine the causes and mechanisms of rare diseases and develop methods to prevent, diagnose, and treat these conditions. Subsequent sections of this chapter discuss the varied causes of rare diseases and examine in broad terms the range of available preventive, diagnostic, and treatment strategies for diverse rare diseases. The last section considers the impact of a rare condition on patients, families, and the broader community and recognizes the efforts by patients, families, and advocacy groups to try, in turn, to have an impact on the disease and those affected by it. Reflecting the large number of rare diseases, their great vari-

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