[1] 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.
[3][4] The saying is a warning against the statistical base rate fallacy where the likelihood of something like a disease among the population is not taken into consideration for an individual.
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 (c. 85 BC), "the striking and the novel stay longer in the mind."
[5] 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 zebra was adopted across the world as the EDS mascot to bring the patient community together and raise awareness.