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For several decades, borreliosis was synonymous with relapsing fever. Since the discovery of the agent of
Lyme disease at the beginning of the 1980s, the term borreliosis now covers both relapsing fever and
Lyme borreliosis. The relapsing fevers form a group of similar diseases that differ from each other, principally, in the different arthropod hosts used by the spirochete as a reservoir and vector. Relapsing fever may be transmitted either by lice (louse-borne relapsing fever) or by soft ticks of the genus Ornithodoros (tick-borne relapsing fever). For several years, reports on relapsing fever have been rare or nonexistent; however, louse-borne and tick-borne relapsing fever still occur. The
disease is most probably confined to parts of Africa, Asia, and the Americas. For a deeper understanding of relapsing fever, recommended is a reading of the excellent monograph Borrelia by Oscar Felsenfeld.1
Lyme borreliosis is reported from those areas of the world where hard ticks of the genus Ixodes, which are the principal vectors of
Lyme borreliosis in the Northern Hemisphere, occur. There is also speculation about the presence of
Lyme borreliosis in the tropics.