Lyme borreliosis is a zoonosis: its causative agent, B: burgdorferi, circulates between ticks and a large range of vertebrates. Identification of the hosts which are responsible for the infection of the vectors is extremely important to determine the potential risk of infection in an habitat. Various small mammals and bird species are considered reservoirs for the Lyme disease spirochetes. Grey and red squirrels, hedgehogs as well as hares and rabbits can develop an infection and transmit B. burgdorferi sensu lato to feeding ticks. In Eurasian endemic areas, many different Borrelia species circulate between ticks and vertebrate hosts. Studies have shown that European and Asian genospecies are associated with specific groups of vertebrate hosts, such as B. valaisiana and B. garinii with birds, B. afzelii with small mammals and B. burgdorferi ss and B. afzelii with red squirrels. However, such associations are not always observed as in Japan where B. garinii, B. afzelii and unidentified Borrelia species are found in small mammals. Some enzootic cycles involving tick species which do not feed at all on humans or which rarely feed on humans have been described in Europe and USA. It is likely that many existing enzootic foci have yet to be discovered. The circulation of B. burgdorferi in silent foci does not have important implications for human health, but it demonstrates the complexity of the ecology of this microorganism and the variety of ecological niches this spirochete can occupy.