[The concept of emerging disease].

To avoid misinterpretations one should substitute the ambiguous notion of 'new disease' with 'emerging disease'. A disease can be classified emergent in at least five different historical situations; 1) it existed before it could be first identified but was overlooked from a medical point of view because it could not be conceptualized as a nosological entity; 2) it existed but was not noticed until a quantitative and/or qualitative change in its manifestations; 3) it did not exist in a particular region of the world before its introduction from other regions; 4) it never existed in a human population but only in an animal population; 5) it is completely new--the triggering germ and/or necessary environmental conditions did not exist prior to the first clinical manifestations. A series of historical examples illustrate this classification.