Tick-borne encephalitis (TBE, in French MEVE) is a viral disease usually transmitted to man through an infected tick belonging to the genus Ixodes. Every year about 135 cases of TBE are reported to the Swiss Federal Office of Public Health. Emerging in north-east of Switzerland in the seventies, endemic areas have been slowly spreading. Actually 18 cantons out of 26 have been reached. In the western part of Switzerland, spared until not long ago, new endemic foci are spreading toward the lake of Geneva and recently in Valais. The precise exposure history of human cases, as well as the evaluation of collected tick samples allow the detection of endemic foci which are discontinuous and of variable size. New and more efficient methods are needed to monitor the changing epidemiology of TBE.