[Concept maps as a tool for the diagnosis of rare diseases].

Rare diseases are a real public health problem for hospitals and also for primary care. We describe some metaphor-based diagnosis procedures, such as: "When you hear hoof beats don't always think horses, sometimes they could be zebras", or that one about the antiquarian who recognised a museum masterpiece while walking in the Rastro (Madrid). The "lightning diagnoses" by Skoda are an important historic record.