Lyme borreliosis is now occurring on several continents where its causative agent, Borrelia burgdorferi, is maintained and transmitted by ticks of the "Ixodes ricinus complex" namely I. dammini, I. pacificus, and possibly I. scapularis in North America, I. ricinus In Europe, and I. persulcatus in Asia. Because all developmental stages of these ticks feed on a large variety of hosts including humans, the vector/host relationships of this spirochete is highly complex as indicated by the voluminous literature reviewed in this article. The association of B. burgdorferi with ticks parasitizing exclusively rabbits and birds, suggests that the geographic distribution of this agent may be far greater than assumed and may include areas where the disease in humans is absent. Finally, the persistence of the Lyme disease spirochete in the midgut of its tick vectors and its invasion of other tissues during the ticks' feeding, are unique and differ from the behavior of all other arthropod-borne borreliae.