[Lyme borreliosis, emergent disease linked with the environment]

After a short historical presentation of the discovery of the pathogen and its vector, the authors present the current data on bacterial and acarologic taxonomy. Then they describe their results to assess the mechanisms of circulation of the bacteria in the forests of Ile-de-France, particularly in the forest of Rambouillet. The combined study of abundance and infection frequency of the vectors, small mammals and cervids leads to the characterization of periods and areas of higher risk. The risk periods correlate with high density of I. ricinus nymphs. The risk areas correspond to those of high density of cervids. The role of reservoir of small mammals is confirmed, to the one of large mammals, so debated, is clearly demonstrated.