Zebras

Came across this at work today:

Zebra: The term "zebra" in medicine does not refer to the striped African animal but to an unlikely diagnostic possibility. It comes from an old saying 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, tuberculosis is a "zebra." For another example, following the discovery of West Nile fever in New York City in September, 1999, the Centers for Disease Control and Prevention warned doctors to expect more infectious disease "zebras" (diseases due to rare microbes).