What Is a Rare Disease, Exactly?

I am Dr Marshall Summar, Chief of Genetics and Metabolism at Children's National Medical Center.

A question that I am often asked is: What exactly is a rare disease? We hear about them in the media. We hear about all the new drug therapies being developed, but there is a lot of confusion over what exactly we are talking about.

There is a practical definition, and what I would call an "official designation." The US Food and Drug Administration (FDA) defines a rare disease as occurring in less than 200,000 people in the United States. That means that as few as five people, or as many as 200,000, could have a rare disease. Obviously, such conditions as asthma or diabetes wouldn't qualify as rare diseases because many more than 200,000 people are affected with these conditions.

Some rare diseases are better known. Down syndrome is at the upper range of prevalence for a rare disease. Phenylketonuria (the metabolic disorder that we screen newborns for) is also considered a rare disease.

But there are other rare diseases that you might not think about when we say "rare disease." Many cancers are developing into rare diseases. With increased genetic technology and diagnosis, what were once common groups of cancers are subdividing into smaller and smaller groups. For example,