Zebra is the American medical slang for arriving at a surprising, often exotic, medical diagnosis when a more commonplace explanation is more likely. It is shorthand for the aphorism coined in the late 1940s by Theodore Woodward, professor at the University of Maryland School of Medicine, who instructed his medical interns: "When you hear hoofbeats behind you, don't expect to see a zebra". Since horses are common in Maryland while zebras are relatively rare, logically one could confidently guess that an animal making hoofbeats is probably a horse. By 1960, the aphorism was widely known in medical circles.
Medical novices are predisposed to make rare diagnoses because of (a) the availability heuristic ("events more easily remembered are judged more probable") and (b) the phenomenon first enunciated in Rhetorica ad Herennium (circa 85 BC), "the striking and the novel stay longer in the mind." Thus, the aphorism is an important caution against these biases when teaching medical students to weigh medical evidence. 
Diagnosticians have noted, however, that "zebra"-type diagnoses must nonetheless be held in mind until the evidence conclusively rules them out:
In making the diagnosis of the cause of illness in an individual case, calculations of probability have no meaning. The pertinent question is whether the disease is present or not. Whether it is rare or common does not change the odds in a single patient. ... If the diagnosis can be made on the basis of specific criteria, then these criteria are either fulfilled or not fulfilled. — A. McGehee Harvey, James Bordley II, Jeremiah Barondess
Comparable slang for an obscure and rare diagnosis in medicine is fascinoma.
Necrotic skin lesions in the United States are often diagnosed as loxoscelism (recluse spider bites), even in areas where Loxosceles species are rare or not present. This is a matter of concern because such misdiagnoses can delay correct diagnosis and treatment.
Ehlers-Danlos syndrome is considered a rare condition and its sufferers are known as medical zebras. The zebra was adopted across the world as the EDS mascot to bring the patient community together and raise awareness.
Other medical aphorisms
- Sutton's law – perform first the diagnostic test expected to be most useful
- Occam's razor – select from among competing hypotheses the one that makes the fewest new assumptions
- Leonard's law of physical findings – it's obvious or it's not there
- Hickam's dictum – "Patients can have as many diseases as they damn well please"
- Samuel Gee — author of Medical lectures and aphorisms (1902)
- James Alexander Lindsay — author of Medical axioms, aphorisms, and clinical memoranda (1924)
- Maimonides — Commentary on the aphorisms of Hippocrates and medical aphorisms of Moses (12th century)
- Sotos (2006) page 1.
- Sotos (2006) page 1. Woodward's original version was: "Don't look for zebras on Greene Street," the street on which the University of Maryland medical campus is sited.
- Imperato (1979) pages 13, 18.
- Dundes, Lauren, Michael B. Streiff, and Alan Dundes. "" When You Hear Hoofbeats, Think Horses, Not Zebras": A Folk Medical Diagnostic Proverb." PROVERBIUM 16 (1999): 95-104.
- Sotos (2006) page 7.
- Harvey (1979) page 15.
- Vetter, Richard S. (2008). "Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomations" (PDF). Journal of Arachnology. 36: 150–163. doi:10.1636/rst08-06.1.
- "Why the zebra? – The Ehlers-Danlos Support UK". Retrieved 2021-10-28.
- Sotos (2006) page 15.
- Harvey, A. M.; et al. (1979). Differential Diagnosis (3rd ed.). Philadelphia: W.B. Saunders.
- Imperato, Pascal James (1979). Medical Detective. New York: Richard Marek. ISBN 0-399-90058-6.
- Sotos, John G. (2006) . Zebra Cards: An Aid to Obscure Diagnoses. Mt. Vernon, VA: Mt. Vernon Book Systems. ISBN 978-0-9818193-0-3.