Many students in medical school are taught “when you hear hoofbeats, think of horses not zebras.” These were Theodore Woodard's instructions to his medical interns at the University of Maryland in the 1940s. This refers to the idea that a doctor’s assumption about a diagnosis should rely on what is most commonly simple, rather than what might be novel or unusual. In regards to the phrase, horses are much more common than zebras, so naturally if you hear hoofbeats you can guess that they probably come from a horse rather than a zebra. This is now a widely used saying in the medical profession often referencing "zebras" when a patient has a rare condition.
This illustration makes fun of the precautions that have been put into place because of this rule and how doctors have a difficult time seeing zebras. In the real world, the effect of this rule has the chance of leading to unfortunate misdiagnoses or the dismissal of "zebras" and their medical problems.