The emergence of
Lyme borreliosis as the most prevalent arthropod
disease of humans in the temperate northern hemisphere has resulted in renewed interest in human babesiosis, transmitted by the same tick vectors. The advent of new molecular tools has made possible a reappraisal of the main parasites involved (Babesia divergens in Europe and Babesia microti in the USA). B. divergens is probably restricted to European cattle, though there are several nearly identical species. B. microti occurs as a world-wide species complex rather than as a single species, and although both phenotypic and genotypic features lend support to suggestions that zoonotic B. microti may occur in Europe, convincing medical evidence is lacking. Comparative biology should support genetic data in taxonomic studies of these parasites.