The International Council of Scientific Unions and the Intergovernmental Panel on Climate Change established by the World Meteorological Organization and the United Nations Environmental Programme, have estimated that by the year 2100, average global temperatures will have risen by between 1.0 and 3.5 ℃. Important ecological changes may come about in the future due to global warming. Globally, 1998 was the warmest year and the 1990s the warmest decade on record. The distribution and seasonality of diseases that are transmitted by insects or ticks, are very likely to be affected by climate change. Such increases in temperatures in Europe might allow the establishment of tropical and semitropical vector species, permitting transmission of diseases in areas where low temperatures have hitherto prevented their over–wintering. Kovats et al. (1999) have pointed out that for the last few decades, Europe has experienced significant warming and this is likely to continue. A change in the distribution of important vector species may be among the first signs of the effect of global climate change on human health. Indeed, there is evidence that the distribution of tick vectors in Sweden has expanded to the north between 1980 and 1994 as reported by Lindgren et al. (2000), and that this is consistent with observed changes in climate. The Swedish study indicated that the reported northern shift in the distribution limit of ticks was related to fewer days during the winter seasons with low minimum temperatures below - 12 ℃.