It really is based on prevalence, so it is defined as a disorder that affects fewer than 1 in 2,000 persons. But by their very nature, most rare disorders are in fact severe. Many are life-threatening, many are debilitating. So just by defining it as 1 in 2,000, you recognize that most of these disorders are in fact fairly severe disorders.
On June 3rd, 2010. See this statement in context.