Each rare disease community is represented by a color. For example, the EDS community is represented by the Turquoise Zebra; the Marfan Syndrome community is represented by the Red Zebra; the rare cancer community is represented by the Pink Zebra, etc. Therefore, the rainbow zebra represents all identified rare conditions.
While the rainbow zebra represents all 7000 rare conditions that have currently been identified. There are several rare diseases that have yet to be identified. These conditions are represented by the black and white zebra. Therefore, the black and white zebra represents all the undiagnosed rare conditions.
"Patients can have as many diseases as they damn well please"
(John Hickam, MD, Duke University, circa 1950)
The Zebra Aphorism has lead the medical community to adopt the Hickam's Dictum, the thinking that it is statistically more likely that a patient has several common diseases rather than a single rare disease. Furthermore, this thinking emphasizes that even when statistically unlikely, patients can prove to have multiple diseases.
Why are so many rare diseases not well-known by the average medical professional?
Rare Disease Awareness
Because the zebra represents ALL rare conditions, it is important to understand the variety of ways rare disease groups differentiate themselves from each other, while embracing their zebra pride.
Another rare disease definition for the rainbow and black & white zebra has taken root. There are 7000 rare diseases currently identified, and several rare diseases being discovered on a regular basis. There are 566 medications currently in development for patients with rare diseases. Once a rare disease acquires a drug that is officially indicated for it, that rare disease community is represented by the rainbow zebra. Those rare disease communities that do not yet have any medications indicated for them are subsequently represented by the black and white zebra.
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"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. McGhee Harvey, James Bordley II, Jeremiah Barondness)
Earlier Versions of the Zebra Aphorism include:
"When you hear hoofbeats behind you, don't expect to see a zebra."
"Don't look for zebras on Greene Street."