An undiagnosed disease means that the medical community cannot assign the symptoms to one particular disease biology, or give it a clear prognosis. Frequently these conditions present a complex set of symptoms and it is difficult to find an underlying diagnosis, because fewer than 200,00 people in the United States are known to be affected. In these cases, a body of knowledge necessary to treat the condition does not yet exist. More than 25 million Americans and their families are part of this group. Many are asked to participate in research studies focused on understanding their condition.