There are more than 7000 rare diseases, also called orphan diseases. How does one decide the threshold for considering a disease to be rare? It depends. One study found more than 296 definitions of “rare disease.” In Europe, a disease is typically considered rare if it affects fewer than 1 in 2000 people, but the most widely used definitions place rare diseases as those that affect 4 or 5 out of 10,000 people. But in today's world of population growth and virtual communities, in many ways rare diseases are becoming common.