(Medical slang terminology)
The health care profession has, like most other fields, an internal terminology of its own, used by its practitioners to communicate among each other with a combination of dark humor and efficiency. As such, there are many terms used to describe categories of situations, conditions, or patients which are used, as hackers use terms in the Jargon File, as a distinct vocabulary
Zebra, in medical terminology, refers to a rare condition or situation. It can refer to either the patient with the condition or the condition itself. (e.g.: "This guy has a real zebra" or"This guy's a zebra")
The origin of the term is shrouded in mystery, but most likely is derivative of a common rule of medical practice:
"When you hear hoofbeats, don't look for zebras."
This is a simple way of saying to look for common things to occur commonly, and rare things to occur rarely.
"Zebras" are the prized discoveries of the intern or resident who correctly diagnoses one and can then stump the other residents, or better yet, the staff physician with a case study during medical rounds. They are the bane of the unlucky doctor who treats the apparent condition only to discover that the hoofbeats did indeed belong to a zebra and not the condition that was diagnosed.
There has been very occasional use of the term "Unicorn" for the almost unheard of case that may come along once in a lifetime - a much more rare occurence than the "zebra" case. This is the one that will likely result in getting written up in a medical journal or the discovery of a new disease (like we need more). The use of this term, however, is as rare as its namesake, and deserves only brief mention in this context.
See also Gomer - Ticks and fleas - Turf