In medical school, many doctors are taught the old saying, “When you hear hoof beats, think horses, not zebras.” This saying means that doctors should generally consider the most likely possibility first when making a diagnosis.

But what do you do when it is a zebra?

What makes a disease rare? In the United States, the Orphan Drug Act defines a disease as rare if it affects fewer than 200,000 Americans or less than one in 2,000 people. A disease is considered ultra-rare if it affects less than one in 50,000 people.