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Borrelia burgdorferi sensu lato is a bacterial complex of at least 10 species, most of which are non pathogenic for humans. Non pathogenic species seem to be more vector-specific, their distribution being restricted to that of this vector. In opposition, the three species pathogenic for humans, Borrelia burgdorferi sensu stricto (B.b.s.s.), Borrelia garinii (B.g.), and Borrelia afzelii (B.a.), are transmitted by several species of ticks more diversified in their host choice. Each of these three species is associated with a preferential organotropism: articulation for B.b. s.s., neurologic system for B.g. and skin for B.a. Strains belonging to these three species are much more frequently isolated from vectors and have a much larger area of expansion than the non pathogenic species. Indeed, their geographical distribution corresponding to that of their vector comprises one or two continents. B.g. and B.a., transmitted by Ixodes ricinus and Ixodes persulcatus, are spread throughout Eurasia. B.b. s.s. is the only species spread on two continents separated by an Ocean: North America (vectors: I. scapularis and I. pacificus) and Europe (I. ricinus). Combining of epidemiological data and molecular analyses shows that B.b. s.s. probably originated in America and later on, very recently (after 1492), migrated to the European continent.