of the relapsing fever group (RFG) are heterogenous and can be divided mainly into three groups according to vectors, namely the soft-tick-borne relapsing fever (STBRF) , the hard-tick-borne relapsing fever (HTBRF) , the louse-borne relapsing fever (LBRF) , and the avian relapsing fever ones. With respect to the geographical distribution, the STBRF are further subdivided into Old World and New World strains. Except for the Avian relapsing fever group , which cause avian spirochetosis, all the others share infectivity in humans. They are indeed the etiological agent of both endemic and epidemic forms of relapsing fever, causing high spirochaetemia and fever. Vectors are primarily soft ticks of spp. in the STBRF group; hard ticks, notably sp., sp., sp., and sp., in the HTBRF group; and the louse in the TBRF one. A recent hypothesis was supported for a common ancestor of RFG , transmitted at the beginning by hard-body ticks. Accordingly, STBRF switched to use soft-bodied ticks as a vector, which was followed by the use of lice by . There are also new candidate species of , at present unclassified, which are also described in this review.