What makes a disease rare is how prevalent it is—that is, the number of individuals living with it. In the United States, a rare disease is one that fewer than 200,000 people live with. (In other words, 60 per 100,000 individuals.) Around the world, rare diseases are identified and addressed differently. The European Union considers a disease rare if it affects no more than 50 per 100,000 people. The World Health Organization, on the other hand, defines a rare disease as one that strikes fewer than 65 per 100,000 people.