A disease or disorder is defined as rare in the USA when it affects fewer than 200,000 Americans at any given time.
WHY DO ZEBRAS REPRESENT RARE DISEASES?
In medicine, a very unlikely diagnostic possibility. It comes from an old saying used in teaching medical students about how to think logically in regard to the differential diagnosis: 'When you hear hoof beats, think of horses, not zebras.' For example, when someone develops a mild transient cough, a virus infection is the most logical and likely cause, and tuberculosisis a zebra.