Résumé
Although the problem of speciation is a puzzle for evolutionists, species are not mere fantasies. In many cases, it is possible to identify evolutionary entities that deserve to be attributed the name 'species' and that are relevant to medical researchers and decision makers. All approaches to the problem of speciation in pathogens are specific cases of four main concepts (or combinations thereof): biological, phylogenetic, phenetic and phenotypic. Modern genetic concepts and technologies help to juggle these concepts.