Humans inhabiting the Old World and New World share a wide variety of pathogens. Processes that result in the disjunct biogeographic distribution of pathogens with common vertebrate reservoirs or vectors are more difficult to unravel than those influencing the distribution of infections spread only through human-to-human transmission. The origins of species and complexes of tick-borne bacteria are unclear. The agent of Lyme borreliosis may have speciated in the New World following geographical isolation of ticks harboring ancestral spirochetes; the subsequent spread to Europe of B. burgdorferi sensu stricto may have occurred within historical times. Other tick-borne agents, such as the ehrlichiae causing human granulocytic ehrlichiosis, are genetically very similar in the Old World and New World. As the taxonomic distinctions among these related agents of human and veterinary importance appear increasingly blurred, the processes leading to the current discontinuous geographic distributions will also become the source of continuing speculation. Accumulating data suggest an Old World origin for a group of bacteria that include B. elizabethae, a human pathogen first identified from the New World. The potential public health significance of these newly described organisms is undefined, but of international interest as their vertebrate reservoir has been introduced throughout the world.